Remove unused members from WebRtcVoiceEngine and channels

This cleanup removes member variables that were declared but never
used in the implementation, likely as a result of previous
refactorings or the send/receive channel split.

- WebRtcVoiceEngine: removes is_dumping_aec_
- WebRtcVoiceSendChannel: removes unsignaled_frame_transformer_
- WebRtcVoiceReceiveChannel: removes send_codec_spec_

Bug: None
Change-Id: I555ceca1b72f8d0f1d9e79b92945fda6850fc2bf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/455200
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47091}