Disable logging during AddCallback, which initializes too early. (#827)

This causes logging to initialize when static constructors are running.
On Xcode 13, this causes a crash when Firebase App is initialized in
the wrong order.

Author: jonsimantov

app/src/util.cc

@@ -227,13 +227,7 @@ void AppCallback::AddCallback(AppCallback* callback) {
   }
   std::string name = callback->module_name();
   if (callbacks_->find(name) != callbacks_->end()) {
-    LogWarning(
-        "%s is already registered for callbacks on app initialization,  "
-        "ignoring.",
-        name.c_str());
   } else {
-    LogDebug("Registered app initializer %s (enabled: %d)", name.c_str(),
-             callback->enabled() ? 1 : 0);
     (*callbacks_)[name] = callback;
   }
 }