Fix crash when clearing NRF52 BLE bonds (#6609)

* Fix crash before clearing BLE bonds

* Prevent clients re-pairing BLE during factory reset
Clients seem able to re-pair BLE after clearing bonds during factory reset, even after advertising disabled. This seems to primarily occur on Android devices, which seem to more actively maintain the BLE connection.
As a workaround, `NRF52Bluetooth::shutdown` swaps the BLE pairing callback to one which actively rejects new connections.

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Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mesh/NodeDB.cpp

@@ -450,7 +450,6 @@ bool NodeDB::factoryReset(bool eraseBleBonds)
         nvs_flash_erase();
 #endif
 #ifdef ARCH_NRF52
-        Bluefruit.begin();
         LOG_INFO("Clear bluetooth bonds!");
         bond_print_list(BLE_GAP_ROLE_PERIPH);
         bond_print_list(BLE_GAP_ROLE_CENTRAL);