plugins: drop leftover declaration.
plugins: drop leftover declaration.

GstVaapiVideoMemory quark is not needed any more, and the actual
implementation was already removed bfore the merge. i.e. this is
an oversight for a hunk that was not meant to be pushed.

gbeauchesne/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,GStreamer/gstreamer-vaapi