glapi: Remove GL_NV_register_combiners from the dispatch table

glapi: Remove GL_NV_register_combiners from the dispatch table

There is no GLX protocol for these functions.  No open-source Linux
driver has ever supported this extension, and it seems unlikely at
this point that one ever will.  There's no reason to have slots for
these functions in the dispatch table.

The unit tests (GetProcAddress::TableDidntShrink and others) are also updated.

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