AudioCodingModuleImpl: Stop failing artificially for non-Opus encoders

All encoders already handle the "Opus-specific" requests sanely (by
failing nicely), so we don't need extra checks to protect them.

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1527453005

Cr-Commit-Position: refs/heads/master@{#11051}
diff --git a/webrtc/modules/audio_coding/acm2/codec_manager.cc b/webrtc/modules/audio_coding/acm2/codec_manager.cc
index bc507dd..d8ef2bf 100644
--- a/webrtc/modules/audio_coding/acm2/codec_manager.cc
+++ b/webrtc/modules/audio_coding/acm2/codec_manager.cc
@@ -166,7 +166,9 @@
     return false;
   }
 
-  if (CurrentEncoderIsOpus()) {
+  // TODO(kwiberg): This doesn't protect Opus when injected as an external
+  // encoder.
+  if (send_codec_inst_ && IsOpus(*send_codec_inst_)) {
     // VAD/DTX not supported, but don't fail.
     enable = false;
   }
@@ -187,9 +189,5 @@
   return true;
 }
 
-bool CodecManager::CurrentEncoderIsOpus() const {
-  return send_codec_inst_ ? IsOpus(*send_codec_inst_) : false;
-}
-
 }  // namespace acm2
 }  // namespace webrtc