mesa: Remove the generated glapi from source control, and just build it.

mesa: Remove the generated glapi from source control, and just build it.

Mesa already always depends on python to build.  The checked in
changes are not reviewed (because any trivial change rewrites the
world).  We also have been pushing commits between xml change and
regen where at-build-time xml-generated code disagrees with committed
xml-generated code.  And worst of all, sometimes we ("I") check in
*stale* xml-generated code.

Acked-by: Ian Romanick <ian.d.romanick@intel.com>