Revert of Allow more than 2 input channels in AudioProcessing.

webrtc/common_audio/audio_util.cc

Index: webrtc/common_audio/audio_util.cc
diff --git a/webrtc/common_audio/audio_util.cc b/webrtc/common_audio/audio_util.cc
index be67c48f6053990ae5c16458980936532166e8a9..2047295cb9142acb794cad2da4cc688734fd14c2 100644
--- a/webrtc/common_audio/audio_util.cc
+++ b/webrtc/common_audio/audio_util.cc
@@ -39,13 +39,4 @@
     dest[i] = FloatS16ToFloat(src[i]);
 }
 
-template <>
-void DownmixInterleavedToMono<int16_t>(const int16_t* interleaved,
-                                       int num_multichannel_frames,
-                                       int num_channels,
-                                       int16_t* deinterleaved) {
-  DownmixInterleavedToMonoImpl<int16_t, int32_t>(
-      interleaved, num_multichannel_frames, num_channels, deinterleaved);
-}
-
 }  // namespace webrtc