mesa: Remove warning that geometry shader support is experimental.

mesa: Remove warning that geometry shader support is experimental.

Geometry shader support is now working well, and adequately piglit
tested.  There are just a few piglit failures left to fix.  So there's
no need for an "experimental" warning anymore.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>