P3A: Stop sending old protobuf format message.

With the completion callback issue addressed, skip submission
to the old P3A endpoint. P2A message continue until we've
confirmed adequite numbers on the json endpoint.

Restores brave/brave-browser#23147

components/p3a/brave_p3a_uploader.cc

@@ -88,6 +88,8 @@ void BraveP3AUploader::UploadLog(const std::string& compressed_log_data,
   } else if (upload_type == "p3a") {
     resource_request->url = p3a_endpoint_;
     resource_request->headers.SetHeader("X-Brave-P3A", "?1");
+    // Don't actually send the older format.
+    return;
   } else {
     NOTREACHED();
   }