FFMpeg/libav cleanups

Various small cleanups, mostly build-system related for FFMpeg/libav:

• codec/avcodec: remove avutil version check
  The avutil version is always higher or equal to 55 with the versions of ffmpeg/libav required in configure.
• ffmpeg: remove unnecessary header checks
  When avcodec is found, it does not really add much value to check for the avcodec header. avutil is required by it, so checking for that is not needed either.
  For avformat, avcodec is required, therefore checking for avcodec and avutil headers is not needed there either.
  Conditionally including those in avcommon_compat.h does not seem needed at all, as it won't compile without these headers anyway.