The fs_hz member variable is going away too, being replaced by a
method in the AudioDecoder interface. If we ever end up needing the
RTP sample rate here, a method ought to be the right solution for that
too.
BUG=webrtc:5801
Review URL: https://codereview.webrtc.org/1907183002
Cr-Commit-Position: refs/heads/master@{#12462}