vout: remove unncessary lock in vout_HoldPicture()

picture_Hold() is reentrant anyway. The vout picture lock protects
the decoder pool. The decoder pool may be modified by picture_Release()
but not by picture_Hold().

src/video_output/video_output.c

@@ -458,11 +458,7 @@ void vout_ReleasePicture(vout_thread_t *vout, picture_t *picture)
  */
 void vout_HoldPicture(vout_thread_t *vout, picture_t *picture)
 {
-    vlc_mutex_lock(&vout->p->picture_lock);
-
     picture_Hold(picture);
-
-    vlc_mutex_unlock(&vout->p->picture_lock);
 }
 
 /* */