There's slots in the glapi static dispatch table (which is still
arguably ABI) which we need to preserve, but we can stop exposing the
extension string or doing anything in the added functions.
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17425>