CodecManager: Eliminate the stereo_send_ member

It can be computed from other members, notably the current encoder's
number of channels.

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1423803007

Cr-Original-Commit-Position: refs/heads/master@{#10585}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: ed8275a44fc13015e6784bf0aba942e836f8d4b7
2 files changed