qtmux: Only write timecode trak for video
qtmux: Only write timecode trak for video

Recent changes in ccextractor were attaching timecode meta to the closed
caption track. We shouldn't write timecode information for the closed
caption trak.

GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good