lavfi/avf_showspectrum: do not push the frame at EOF.

It is always identical to the last pushed frame.
The samples in the last incomplete window were ignored,
this is unchanged.
Possible enhancement: pad the last incomplete window with
silence.

libavfilter/avf_showspectrum.c

@@ -281,8 +281,6 @@ static int request_frame(AVFilterLink *outlink)
         ret = ff_request_frame(inlink);
     } while (!s->req_fullfilled && ret >= 0);
 
-    if (ret == AVERROR_EOF && s->outpicref)
-        push_frame(outlink);
     return ret;
 }