Previously cogl_gst_video_sink_attach_frame returned the last layer
used by the sink. This can also be retrieved via
cogl_gst_video_sink_get_free_layer so I don't think it's necessary to
return it here and it seems kind of out of place.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 0805f3e9db715fc2c97b7e60d4f3a25e7fa598fc)