Cleanup: Iterative removal of matured deprecated symbols (Batch 3)

This iteration removes 8 deprecated symbols that have been matured for
at least 3 months and have zero usage in downstream projects.

Summary of removals:
1.  VoiceEngineInterface::send_codecs()
2.  VoiceEngineInterface::recv_codecs()
3.  IvfFileWriter::Wrap(FileWrapper, ...)
(2 failures)
6.  StunMessage::GetString()
7.  PortAllocator::SubscribePortReady
8.  PortAllocator::SubscribePortsPruned
9.  PortAllocator::SubscribeCandidatesReady
10. PortAllocator::SubscribeCandidateError

Also updates GEMINI.md and g3doc/todo/remove_deprecated.md with
high-precision CodeSearch filters (usage: and func:) for future audits.

Bug: webrtc:465197113
Change-Id: Ie5490e22df06754243c8b8dbf65df78716200fa3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/469660
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47621}