mesa: Remove GL_EXT_clip_volume_hint

As far as I can tell, no driver has enabled this extension since c6499a7
back in 2007.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

docs/relnotes/9.2.html

@@ -63,6 +63,8 @@ Note: some of the new features are only available with certain drivers.
 
 <ul>
 <li>Removed d3d1x state tracker (unused, unmaintained and broken)</li>
+<li>Removed GL_EXT_clip_volume_hint because no driver had enabled it since
+2007.</li>
 </ul>
 
 </div>