Remove RentACodec::GetEncoderStack

Callers can just remember the return value of
RentACodec::RentEncoderStack instead.

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1612713002

Cr-Original-Commit-Position: refs/heads/master@{#11340}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 32be07bc36813d351c3555c502a819d3dd172d07
5 files changed