Drop support for OpenGL denylist on X11

[Stephan Bergmann]

Just a heads up about <https://gerrit.libreoffice.org/c/core/+/131943> 
"Drop support for OpenGL denylist on X11", which would drop that 
denylist and the underlying glxtest machinery, for the reasons stated 
there.  Please speak up if you see any issues with that.