simple-dmabuf-v4l: Remove incorrect assert

According to v4l2 documentation, DQBUF always clears FLAG_DONE, so
this assert can be expected to fire 100% of the time.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

clients/simple-dmabuf-v4l.c

@@ -457,8 +457,6 @@ dequeue(struct display *display)
 		return -1;
 	}
 
-	assert(buf.flags & V4L2_BUF_FLAG_DONE);
-
 	return buf.index;
 }