mpegvideoparse: Revert "fix crash"

This reverts commit 91210831ee672343a296f31357144359d5c2e768.

Such explicit reset should not be needed as it is arranged for by
the baseclass in unison with monitoring for a new frame by subclass.

As such it might wrongfully hide something else going on ...

See #674073.

gst/videoparsers/gstmpegvideoparse.c

@@ -550,7 +550,6 @@ gst_mpegv_parse_handle_frame (GstBaseParse * parse,
 end:
   if (fsize > 0) {
     ret = TRUE;
-    mpvparse->last_sc = -1;
   } else if (GST_BASE_PARSE_DRAINING (parse)) {
     fsize = buf_size;
     ret = TRUE;