matroska-demux: remove redundant variable
matroska-demux: remove redundant variable

matsu/gst-plugins-good,matsu/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,matsu/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,matsu/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good