glx: remove support for non-multithreaded platforms

Implicitly required for a while, although commit 9385c592c68 (mapi:
remove u_thread.h) was the one that put the final nail on the
coffin.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
4 files changed