ffplay: remove no longer necessary codec flush

It was introduced in c2e8691c07, but since we no
longer no longer provide a custom get_buffer callback, the original cause of
the issue is gone.

Signed-off-by: Marton Balint <cus@passwd.hu>

ffplay.c

@@ -1990,7 +1990,6 @@ static int video_thread(void *arg)
             goto the_end;
     }
  the_end:
-    avcodec_flush_buffers(is->video_st->codec);
 #if CONFIG_AVFILTER
     avfilter_graph_free(&graph);
 #endif