Remove the remains of GLX_MESA_multithread_makecurrent
This hasn't actually been exported for a while. I think I probably broke this in
commit 63a6b719d98fb1ad58ae93c2de859e6d4bfa8b8b
Author: Adam Jackson <ajax@redhat.com>
Date: Tue Dec 5 11:10:09 2017 -0500
glx: GLX_MESA_multithread_makecurrent is direct-only
in which I made it no longer default to having client support, but failed to instruct dri{2,3,sw} to enable it. In any case, it was never widely used, there is no EGL equivalent, and we've had zero complaints about it getting nerfed.
cc @anholt