[ffmpeg] update the previous roll to remove vp9, sbc encode

This CL updates the previous ffmpeg roll using changes in fxr/817552.
Specifically, support for sbc encode and vp9 decode, neither of which
are needed, have been removed.

Change-Id: Icb9436ed3059b96f45d805f5c74842d23c37b2d2
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/ffmpeg/+/817272
Reviewed-by: Petr Hosek <phosek@google.com>