avcodec/avcodec: Remove redundant assert

It is now checked by FATE that no encoder capable of flushing
uses frame threads, so this now redundant runtime check can
be removed.

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

libavcodec/avcodec.c

@@ -390,9 +390,6 @@ void avcodec_flush_buffers(AVCodecContext *avctx)
                    "that doesn't support it\n");
             return;
         }
-
-        // We haven't implemented flushing for frame-threaded encoders.
-        av_assert0(!(caps & AV_CODEC_CAP_FRAME_THREADS));
     }
 
     avci->draining      = 0;