Clean up the NetEqFactory API.

This CL decouples NetEqFactory and AudioDecoderFactory.
AudioDecoderFactory is used in more places than just inside of NetEq, so
decoupling these makes sense.

Bug: webrtc:11005
Change-Id: I78dd856e4248e398e69a65816b062ef30555b055
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161005
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29961}

modules/audio_coding/neteq/default_neteq_factory.cc

@@ -28,12 +28,4 @@ std::unique_ptr<NetEq> DefaultNetEqFactory::CreateNetEq(
                                       controller_factory_));
 }
 
-std::unique_ptr<NetEq> DefaultNetEqFactory::CreateNetEq(
-    const NetEq::Config& /*config*/,
-    Clock* /*clock*/) const {
-  RTC_NOTREACHED() << "Calling CreateNetEq without an AudioDecoderFactory on "
-                      "DefaultNetEqFactory is not supported.";
-  return nullptr;
-}
-
 }  // namespace webrtc