Cleanup: Remove matured deprecated symbols with zero external usage

This CL removes a batch of deprecated symbols from the WebRTC API and
internal headers. All removed symbols have zero confirmed usage in
downstream projects and have been deprecated for at least 3 months.

Symbols retired in this iteration:
   * SdpAudioFormat::Parameters
   * EncodedImage::IsAtTargetQuality / SetAtTargetQuality
   * kRtpExtensionGenericFrameDescriptor00
   * kRtpExtensionGenericFrameDescriptor02

Bug: webrtc:41480926
Change-Id: Id501c5779f65eb2bc5eff88c43c0a2a21f532893
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/467500
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47565}
diff --git a/api/audio_codecs/audio_format.h b/api/audio_codecs/audio_format.h
index f7daf76..d390b19 100644
--- a/api/audio_codecs/audio_format.h
+++ b/api/audio_codecs/audio_format.h
@@ -13,7 +13,6 @@
 
 #include <stddef.h>
 
-#include <map>
 #include <string>
 
 #include "absl/strings/string_view.h"
@@ -26,9 +25,6 @@
 
 // SDP specification for a single audio codec.
 struct RTC_EXPORT SdpAudioFormat {
-  using Parameters [[deprecated("Use CodecParameterMap")]] =
-      std::map<std::string, std::string>;
-
   SdpAudioFormat(const SdpAudioFormat&);
   SdpAudioFormat(SdpAudioFormat&&);
   SdpAudioFormat(absl::string_view name, int clockrate_hz, size_t num_channels);
diff --git a/api/video/encoded_image.h b/api/video/encoded_image.h
index a934347..1b89ac2 100644
--- a/api/video/encoded_image.h
+++ b/api/video/encoded_image.h
@@ -209,16 +209,6 @@
   const uint8_t* begin() const { return data(); }
   const uint8_t* end() const { return data() + size(); }
 
-  // Returns whether the encoded image can be considered to be of target
-  // quality.
-  [[deprecated]] bool IsAtTargetQuality() const { return at_target_quality_; }
-
-  // Sets that the encoded image can be considered to be of target quality to
-  // true or false.
-  [[deprecated]] void SetAtTargetQuality(bool at_target_quality) {
-    at_target_quality_ = at_target_quality;
-  }
-
   // Returns whether the frame that was encoded is a steady-state refresh frame
   // intended to improve the visual quality.
   bool IsSteadyStateRefreshFrame() const {
@@ -317,8 +307,6 @@
   // https://w3c.github.io/webrtc-pc/#dom-rtcrtpreceiver-getcontributingsources
   RtpPacketInfos packet_infos_;
   bool retransmission_allowed_ = true;
-  // True if the encoded image can be considered to be of target quality.
-  bool at_target_quality_ = false;
   // True if the frame that was encoded is a steady-state refresh frame intended
   // to improve the visual quality.
   bool is_steady_state_refresh_frame_ = false;
diff --git a/g3doc/todo/remove_deprecated.md b/g3doc/todo/remove_deprecated.md
index dd52c2f..249bc0a 100644
--- a/g3doc/todo/remove_deprecated.md
+++ b/g3doc/todo/remove_deprecated.md
@@ -13,7 +13,7 @@
 A symbol is a candidate for removal only if it meets **all** of the following:
 
 1. **Zero Usage:** No references found in WebRTC internals or downstream
-   projects. Chromium/WebKit.
+   projects.
 2. **Maturity:** The `[[deprecated]]` tag was added at least **3 months** ago.
 3. **Batch Size:** No more than **10 symbols** are processed in a single
    iteration.
@@ -35,11 +35,14 @@
 4. **Downstream Audit (CodeSearch):** For each matured symbol, perform broad
    searches in downstream projects (outside `third_party/webrtc`).
    - **External Usage:** `cs "content:<SymbolName> -file:stable/webrtc"`
-   - **Chromium Overrides:**
-     `cs "content:<SymbolName> file:chromium -file:third_party/webrtc"`
+   - **Search Optimization:** To minimize false positives with common names
+     (like `kPlanB` or `local_ssrc`), prioritize searching for qualified usage
+     (e.g., `cs "content:SdpSemantics::kPlanB"` or `cs "content:::kPlanB"`) or
+     structural access (e.g., `cs "content:.local_ssrc"`) before performing
+     broad searches for the bare symbol name.
    - *Note:* Pay special attention to virtual methods. If a symbol is overridden
-     in Chromium (e.g., in Blink's `RTCDTMFSenderHandler`), removing it from the
-     WebRTC base class will break the Chromium build.
+     in a downstream project, removing it from the WebRTC base class will break
+     the downstream build.
 5. **Iteration Stop:** Continue the audit process until exactly **10 symbols**
    have been confirmed to have zero results. Once 10 are found, stop the search.
 6. **Documentation:** List the 10 selected symbols, their locations, and their
@@ -61,7 +64,8 @@
    `rtc_unittests`, `rtc_pc_unittests`, `peerconnection_unittests`).
 5. **Verification:** Run the relevant test suites and ensure 100% pass rate.
 6. **Gerrit Upload:** Submit the changes for review. The CL description should
-   not contain any information about downstream projects.
+   refer to "downstream projects" generally and MUST NOT contain the specific
+   names of these projects.
 
 ______________________________________________________________________
 
diff --git a/modules/rtp_rtcp/include/rtp_rtcp_defines.h b/modules/rtp_rtcp/include/rtp_rtcp_defines.h
index a8b2354..fac8d06 100644
--- a/modules/rtp_rtcp/include/rtp_rtcp_defines.h
+++ b/modules/rtp_rtcp/include/rtp_rtcp_defines.h
@@ -78,11 +78,7 @@
   kRtpExtensionRepairedRtpStreamId,
   kRtpExtensionMid,
   kRtpExtensionGenericFrameDescriptor,
-  kRtpExtensionGenericFrameDescriptor00 [[deprecated]] =
-      kRtpExtensionGenericFrameDescriptor,
   kRtpExtensionDependencyDescriptor,
-  kRtpExtensionGenericFrameDescriptor02 [[deprecated]] =
-      kRtpExtensionDependencyDescriptor,
   kRtpExtensionColorSpace,
   kRtpExtensionVideoFrameTrackingId,
   kRtpExtensionCorruptionDetection,