ffmpeg: remove misleading and incorrect warning messages

ffmpeg: remove misleading and incorrect warning messages

It is wrong/incorrect in two aspects:
1. The pixel format is not enough to guarantee that the resulting file
   will be any more compatible with media players.
2. Media players not supporting higher profiles are not necessarily
   outdated (in fact this is simply an arrogant statement that
   libavcodec can handle these particular features).

You could add that there are plenty of other ways to produce widely
incompatible files with ffmpeg, and these don't show any warnings.

What we really want to do here is defaulting to codec profiles that
have wide compatibility, such as main/high for h264. Also, if an
encoder does not accept certain pixfmts, we should automatically
convert them to a pixfmt the encoder can accept. But the existing
message certainly is not appropriate.

It also works for 2 specific encoders only. Extending it for other
cases would result in a lot of special cases, so this is not the
right place.