remove /prefs for nrf factory reset

src/mesh/NodeDB.cpp

@@ -95,6 +95,8 @@ bool NodeDB::resetRadioConfig()
         nvs_flash_erase();
 #endif
 #ifdef NRF52_SERIES
+        FS.rmdir_r("/prefs");
+
         Bluefruit.begin();
 
         DEBUG_MSG("Clearing bluetooth bonds!\n");