Removed the configuration options for multi_channel processing

This CL removes the explicit configuration options for whether
the audio processing should be performed in a multi- or
single-channel manner.
This change means that the choice of whether to do multi- or
single-channel audio processing will be made based on the
content of the audio sent to APM for processing, and the
audio output requested by APM.

Bug: chromium:464314991
Change-Id: I55a53f2565c81a7d7ad2e378f944ffcf07c71f4f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/453000
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47175}