gst/matroska/matroska-mux.*: Don't write another SeekHead which indexes all Clusters to the end of the file. This isn...

Original commit message from CVS:
* gst/matroska/matroska-mux.c: (gst_matroska_mux_reset),
(gst_matroska_mux_start), (gst_matroska_mux_finish),
(gst_matroska_mux_write_data):
* gst/matroska/matroska-mux.h:
Don't write another SeekHead which indexes all Clusters to the end of
the file. This isn't useful for anything and just increases filesize.