Remove check and error message for "MBAFF + !direct_8x8_inference is not implemented"
because this mode does not exist, H.264-2007 says "When frame_mbs_only_flag is
equal to 0, direct_8x8_inference_flag shall be equal to 1."

Originally committed as revision 15369 to svn://svn.ffmpeg.org/ffmpeg/trunk
