PSA: All legacy desktop AEC code will be removed from WebRTC builds.

[Per Åhgren]

We will be removing all code related to the legacy desktop echo canceller AEC (“AEC2”) from the WebRTC codebase, including the code for extended filter and delay agnostic.

The reason is that the legacy desktop AEC no longer fills a role as the new WebRTC desktop echo canceller AEC3 performs much better on desktop platforms, and as the legacy desktop AEC is outperformed on mobile platforms by the WebRTC mobile echo canceller AECm. 

The deprecation will be done in two steps:

* 2 weeks from now, the legacy desktop AEC will be excluded from the default builds

* during the M81 release cycle, the code will be removed from the WebRTC repository