broadband-modem-mbim: disable parent access tech loading

The parent method would use QCDM to load access tech, but we already
have the access tech reported in MBIM via indications, so no real need
to mix that.

For context, see:
https://bugs.freedesktop.org/show_bug.cgi?id=100000

src/mm-broadband-modem-mbim.c

@@ -3238,6 +3238,8 @@ iface_modem_init (MMIfaceModem *iface)
     iface->setup_charset_finish = NULL;
     iface->load_signal_quality = NULL;
     iface->load_signal_quality_finish = NULL;
+    iface->load_access_technologies = NULL;
+    iface->load_access_technologies_finish = NULL;
 
     /* Create MBIM-specific SIM */
     iface->create_sim = create_sim;