Remove unused private from folly/io/async/test/AsyncUDPSocketSendmmsgTest.cpp

Summary:
`-Wunused-private-field` has identified an unused private field. This diff removes it.

If the code compiles, this is safe to land.

Reviewed By: dtolnay

Differential Revision: D70933471

fbshipit-source-id: 553f8550b15b00d7d1562e71609717ba6b930222

Author: r-barnes

folly/io/async/test/AsyncUDPSocketSendmmsgTest.cpp

@@ -90,7 +90,7 @@ struct TestData {
 
 class UDPAcceptor : public AsyncUDPServerSocket::Callback {
  public:
-  UDPAcceptor(EventBase* evb) : evb_(evb) {}
+  UDPAcceptor(EventBase* /* evb */) {}
 
   void onListenStarted() noexcept override {}
 
@@ -105,9 +105,6 @@ class UDPAcceptor : public AsyncUDPServerSocket::Callback {
     // send pong
     socket->write(client, data->clone());
   }
-
- private:
-  EventBase* const evb_{nullptr};
 };
 
 class UDPServer {