ACM: Move NACK functionality inside NetEq

Negative acknowledgement (NACK) has up to now been implemented in
ACM. But, since NetEq is in charge of the actual packet buffer, it
makes more sense to have the NACK functionlaity in there.

This CL does the following:
- Move nack.{h,cc} and the unit tests from main/acm2 to neteq.
- Move the NACK related code in ACM into NetEq.
- NACK related functions in AcmReceiver are changed to simple
  forwarding APIs.
- Remove unused members in AcmReceiver.
- Remove unused API functions in NetEq.

BUG=webrtc:3520

Review URL: https://codereview.webrtc.org/1410073006

Cr-Commit-Position: refs/heads/master@{#10448}

webrtc/modules/audio_coding/main/audio_coding_module.gypi

@@ -108,8 +108,6 @@
         'acm2/codec_owner.h',
         'acm2/initial_delay_manager.cc',
         'acm2/initial_delay_manager.h',
-        'acm2/nack.cc',
-        'acm2/nack.h',
         'include/audio_coding_module.h',
         'include/audio_coding_module_typedefs.h',
       ],