Clean up in module_common_types.h by removing the unused struct RTPAudioHeader.

By removing it we can in turn (next CL) get rid of RTPTypeHeader, which is a
union that cause some problems.

Bug: none
Change-Id: I9246ecbfe2c8b7eda27497cccbc5f438958b64bf
Reviewed-on: https://webrtc-review.googlesource.com/83985
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23666}

modules/audio_coding/test/target_delay_unittest.cc

@@ -43,8 +43,6 @@ class TargetDelayTest : public ::testing::Test {
     rtp_info_.header.ssrc = 0x12345678;
     rtp_info_.header.markerBit = false;
     rtp_info_.header.sequenceNumber = 0;
-    rtp_info_.type.Audio.channel = 1;
-    rtp_info_.type.Audio.isCNG = false;
     rtp_info_.frameType = kAudioFrameSpeech;
 
     int16_t audio[kFrameSizeSamples];