It is not used, and it is unlikely that a new use case will emerge
anytime soon because the scope of OpenGL contexts are limited due to
the nature of the frontend, VirGL, processing simple commands from the
guest.

Remove the function and ease implementing a new OpenGL backend a little.

Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-Id: <20210219094702.90789-1-akihiko.odaki@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>