Revert "Tests for multi-stream Opus."

This reverts commit 9c31ac23231a3494a794b3ba0a6b018969eaa7aa.

Reason for revert: Breaks downstream project.

Original change's description:
> Tests for multi-stream Opus.
> 
> This CL (mainly) adds bit-exactness tests for multi-stream Opus. The
> tests are in audio_coding_unittest.cc. Some refactoring of
> AcmSendTestOldApi, AcmSenderBitExactnessOldApi is done to make it
> possible. A few checks for "channels \in {1, 2}" are replaced with
> "channels \in {1, 2, 4, 6, 8}" in the WebRTC Opus codec wrapper. A few
> other changes are made to be able to write and read multi-channel WAV
> files.
> 
> The SDP changes are NOT included; as of this CL there is no way to set
> up a multi-stream opus en/de-coder from SDP strings.
> 
> Bug: webrtc:8649
> Change-Id: I1d93a9b8eecc3f6e19896ff2e2ce9b63da77a23c
> Reviewed-on: https://webrtc-review.googlesource.com/c/114883
> Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
> Commit-Queue: Alex Loiko <aleloi@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#26742}

TBR=aleloi@webrtc.org,ossu@webrtc.org

Change-Id: I0ac48b7320d31d3e7af33bf8714c3db6c807b82e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:8649
Reviewed-on: https://webrtc-review.googlesource.com/c/123385
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26747}
diff --git a/modules/audio_coding/acm2/audio_coding_module.cc b/modules/audio_coding/acm2/audio_coding_module.cc
index 4cad577..014aede 100644
--- a/modules/audio_coding/acm2/audio_coding_module.cc
+++ b/modules/audio_coding/acm2/audio_coding_module.cc
@@ -477,9 +477,7 @@
     return -1;
   }
 
-  if (audio_frame.num_channels_ != 1 && audio_frame.num_channels_ != 2 &&
-      audio_frame.num_channels_ != 4 && audio_frame.num_channels_ != 6 &&
-      audio_frame.num_channels_ != 8) {
+  if (audio_frame.num_channels_ != 1 && audio_frame.num_channels_ != 2) {
     RTC_LOG(LS_ERROR) << "Cannot Add 10 ms audio, invalid number of channels.";
     return -1;
   }