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}

webrtc/modules/audio_coding/acm2/codec_manager.h

@@ -53,8 +53,6 @@ class CodecManager final {
 
   bool SetCodecFEC(bool enable_codec_fec);
 
-  bool CurrentEncoderIsOpus() const;
-
  private:
   rtc::ThreadChecker thread_checker_;
   rtc::Optional<CodecInst> send_codec_inst_;