mesa: Remove support for desktop OpenGL GL_EXT_separate_shader_objects

mesa: Remove support for desktop OpenGL GL_EXT_separate_shader_objects

I don't know of any applications that actually use it.  Now that Mesa
supports GL_ARB_separate_shader_objects in all drivers, this extension
is just cruft.

The entrypoints for the extension remain in the XML.  This is done so
that a new libGL will continue to provide dispatch support for old
drivers that try to expose this extension.

Future patches will add OpenGL ES GL_EXT_separate_shader_objects, but
that's a different thing.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>