cogl-gst: Remove the unused cogl_gst_video_sink_get_main_loop
cogl-gst: Remove the unused cogl_gst_video_sink_get_main_loop

This declaration was a leftover from a previous iteration of the
CoglGST patches.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

(cherry picked from commit 53ae6d39efa113e28f5d1177ccb8613ddf28ee43)

Distrotech/cogl,Distrotech/cogl,Distrotech/cogl,Distrotech/cogl