mesa: remove the driverCtx parameter to _mesa_create/initialize_context()

No longer used.
diff --git a/src/mesa/state_tracker/st_context.c b/src/mesa/state_tracker/st_context.c
index aa35341..da361d8 100644
--- a/src/mesa/state_tracker/st_context.c
+++ b/src/mesa/state_tracker/st_context.c
@@ -225,7 +225,7 @@
    memset(&funcs, 0, sizeof(funcs));
    st_init_driver_functions(&funcs);
 
-   ctx = _mesa_create_context(api, visual, shareCtx, &funcs, NULL);
+   ctx = _mesa_create_context(api, visual, shareCtx, &funcs);
    if (!ctx) {
       return NULL;
    }