microbit: Disabled default instantiation of DFUService.

micro:bit operates its own custom service for this purpose.

source/BLE.cpp

@@ -32,7 +32,7 @@ BLE::initImplementation(FunctionPointerWithContext<InitializationCompleteCallbac
     /* Platforms enabled for DFU should introduce the DFU Service into
      * applications automatically. */
 #if defined(TARGET_OTA_ENABLED)
-    static DFUService dfu(*this); // defined static so that the object remains alive
+    //static DFUService dfu(*this); // defined static so that the object remains alive
 #endif // TARGET_OTA_ENABLED
 
     return BLE_ERROR_NONE;