Remove non-functional asynchronous resampling mode.

A few other cleanups, most notably using a sane parameter to specify the
number of channels.

BUG=chromium:469814
R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46729004

Cr-Commit-Position: refs/heads/master@{#8894}
8 files changed