message: [mdns] fix unused variable of aServiceRef
diff: @@ -435,12 +435,13 @@ exit:
  void PublisherMDnsSd::DiscardService(const char *aName, const char *aType, DNSServiceRef aServiceRef)
  {
  + OTBR_UNUSED_VARIABLE(aServiceRef);
  +
  ServiceIterator service = FindPublishedService(aName, aType);
  if (service != mServices.end())
  {
  assert(aServiceRef == nullptr || aServiceRef == service->mService);
  - OTBR_UNUSED_VARIABLE(aServiceRef);
  otbrLogInfo("Remove service ref %p", service->mService);
  