PSA: Removing support for keyboard microphone from WebRTC APM

[Henrik Lundin]

Keyboard microphone support was recently removed from Chrome; see bug and CL. With this change, some functionality in WebRTC's Audio Processing Module (APM) is left unused by Chrome.

We will soon land a CL that removes this unused functionality, including the following in APM's API:

- has_keyboard_ from StreamConfig and the related constructor argument

- AudioProcessing::ChannelLayout, which is no longer of use

- AudioProcessingImpl::ApmCaptureState::KeyboardInfo, which is also no longer needed

See the linked CL for details.

Sincerely,

/Henrik Lundin

[Henrik Lundin]

This is now complete. The change was landed in pieces; see https://webrtc-review.googlesource.com/c/src/+/250221 and https://webrtc-review.googlesource.com/c/src/+/251242.