Delete enum NetEqDecoder.

A trimmed down version is moved to legacy_encoded_audio_frame_unittest.cc
where it's used for test parameterization.

Bug: webrtc:10185
Change-Id: I9abda22f9806b831b6ca4b27d6bcc888285f50f2
Reviewed-on: https://webrtc-review.googlesource.com/c/116961
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26218}

modules/audio_coding/neteq/neteq_unittest.cc

@@ -258,7 +258,6 @@ class NetEqDecodingTest : public ::testing::Test {
   NetEqDecodingTest();
   virtual void SetUp();
   virtual void TearDown();
-  void SelectDecoders(NetEqDecoder* used_codec);
   void OpenInputFile(const std::string& rtp_file);
   void Process();