vda_h264_dec: remove check_format

Since this function cannot work anymore, this patch simply removes it.
Merge branch 'master' of https://github.com/upsuper/ffmpeg-vdadec

* 'master' of https://github.com/upsuper/ffmpeg-vdadec:
  vda_h264_dec: remove check_format

Merged-by: Michael Niedermayer <michaelni@gmx.at>