avcodec/encode: Remove deprecated always-false checks

Now that it is ensured that the old and new channel count/layout
values coincide if the old ones are set, the consistency of the
AVChannelLayout (which is checked before we reach this point)
implies the consistency of the old values, making these checks
here dead code. So remove them.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>