qtmux: Remove warning in the log for mono video
qtmux: Remove warning in the log for mono video

Vanilla mono video was generating a spurious warning into the debug log
that's just misleading. Handle mono caps explicitly to avoid the warning.

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