mesa: remove GL_MESA_program_debug extension

This was never fully fleshed out and hasn't been used.

src/mesa/drivers/common/driverfuncs.c

@@ -134,9 +134,6 @@ _mesa_init_driver_functions(struct dd_function_table *driver)
    driver->BindProgram = NULL;
    driver->NewProgram = _mesa_new_program;
    driver->DeleteProgram = _mesa_delete_program;
-#if FEATURE_MESA_program_debug
-   driver->GetProgramRegister = _mesa_get_program_register;
-#endif /* FEATURE_MESA_program_debug */
 
    /* simple state commands */
    driver->AlphaFunc = NULL;