Drop RtpRtcp unittest dependency on global field trial string

FieldTrialBasedConfig reads config from the global field trial string
ScopedKeyValueConfig adjust the global field trial string
test::ExplicitKeyValueConfig doesnt touch the global field trial string

Bug: webrtc:11926
Change-Id: I8883634fdc7e1bdb63eec9bf38114a3031103839
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/299062
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Auto-Submit: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39683}

modules/rtp_rtcp/source/rtp_rtcp_interface.h

diff --git a/modules/rtp_rtcp/source/rtp_rtcp_interface.h b/modules/rtp_rtcp/source/rtp_rtcp_interface.h
index d49051b..66a29c9 100644
--- a/modules/rtp_rtcp/source/rtp_rtcp_interface.h
+++ b/modules/rtp_rtcp/source/rtp_rtcp_interface.h
@@ -122,8 +122,7 @@
     // done by RTCP RR acking.
     bool always_send_mid_and_rid = false;
 
-    // If set, field trials are read from `field_trials`, otherwise
-    // defaults to  webrtc::FieldTrialBasedConfig.
+    // If set, field trials are read from `field_trials`.
     const FieldTrialsView* field_trials = nullptr;
 
     // SSRCs for media and retransmission, respectively.