Remove dd_function_table::BlendFunc. All drivers now use

dd_function_table:BlendFuncSeparate.  If a driver does not actually
support EXT_blend_func_separate, it can assume that the RGB and alpha
blend functions are the same.

src/mesa/drivers/common/driverfuncs.c

@@ -110,7 +110,6 @@ _mesa_init_driver_functions(struct dd_function_table *driver)
    driver->AlphaFunc = NULL;
    driver->BlendColor = NULL;
    driver->BlendEquation = NULL;
-   driver->BlendFunc = NULL;
    driver->BlendFuncSeparate = NULL;
    driver->ClearColor = NULL;
    driver->ClearDepth = NULL;