matroska: Do not write SegmentUID for WebM mux
matroska: Do not write SegmentUID for WebM mux

WebM spec states SegmentUID is Unsupported. Files produced
with gstreamer without this change will spit an error like
this when passed to mkvalidator:

ERR201: Invalid 'SegmentUID' for profile 'webm' in Info at 192

jhodapp/gst-plugins-good,jhodapp/gst-plugins-good,jhodapp/gst-plugins-good,jhodapp/gst-plugins-good