libs: display: remove libgstvaapi_init_once()
libs: display: remove libgstvaapi_init_once()

It is not required since it can be unrolled in
gst_vaapi_display_class_init()

https://bugzilla.gnome.org/show_bug.cgi?id=788058

ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi