NetEq: Deprecate playout modes Fax, Off and Streaming

The playout modes other than Normal have not been reachable for a long
time, other than through tests. It is time to deprecate them.

The only meaningful use was that Fax mode was sometimes set from
tests, in order to avoid time-stretching operations (accelerate and
pre-emptive expand) from messing with the test results. With this CL,
a new config is added instead, which lets the user specify exactly
this: don't do time-stretching.

As a result of Fax and Off modes being removed, the following code
clean-up was done:
- Fold DecisionLogicNormal into DecisionLogic.
- Remove AudioRepetition and AlternativePlc operations, since they can
  no longer be reached.

Bug: webrtc:9421
Change-Id: I651458e9c1931a99f3b07e242817d303bac119df
Reviewed-on: https://webrtc-review.googlesource.com/84123
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23704}

modules/audio_coding/neteq/neteq.cc

@@ -30,7 +30,7 @@ std::string NetEq::Config::ToString() const {
   ss << "sample_rate_hz=" << sample_rate_hz << ", enable_post_decode_vad="
      << (enable_post_decode_vad ? "true" : "false")
      << ", max_packets_in_buffer=" << max_packets_in_buffer
-     << ", playout_mode=" << playout_mode << ", enable_fast_accelerate="
+     << ", enable_fast_accelerate="
      << (enable_fast_accelerate ? " true" : "false")
      << ", enable_muted_state=" << (enable_muted_state ? " true" : "false");
   return ss.str();