The __DRI_USE_INVALIDATE extension was added in May 11th, 2010 by commit
4258e3a2.  At this point, it's unlikely that anyone's using the
right mix of new and old components to hit this path.  Deleting it
removes an untested code path and cleans up the driver a bit.

Cc: Kristian Høgsberg <krh@bitplanet.net>
Cc: Keith Packard <keithp@keithp.com>