mesa: remove remaining uses of _glthread_GetID()

mesa: remove remaining uses of _glthread_GetID()

It was really only used in the radeon driver for a debug printf.
And evidently, libGL.so referenced it just to work around some sort
of linker issue.

This patch removes the two calls to the function and the function
itself.

Fixes undefined _glthread_GetID symbol in libGL reported by 'nm'.
Though, the missing symbol doesn't cause any issues on my system but
it does cause glxinfo to fail on one of our test systems.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>