mp: remove unused variable outlink in end_frame

Fix warning:
vf_mp.c: In function ‘end_frame’:
vf_mp.c:870: warning: unused variable ‘outlink’
Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>

libavfilter/vf_mp.c

@@ -867,7 +867,6 @@ static void end_frame(AVFilterLink *inlink)
 {
     MPContext *m = inlink->dst->priv;
     AVFilterBufferRef *inpic  = inlink->cur_buf;
-    AVFilterLink *outlink = inlink->dst->outputs[0];
     int i;
     double pts= MP_NOPTS_VALUE;
     mp_image_t* mpi = new_mp_image(inpic->video->w, inpic->video->h);