message: Remove deprecated enable_protocol_5() method in test code
  Reflecting review comments.
diff: @@ -564,8 +564,8 @@ TEST_F(ProtocolHandlerImplTest,
  StartSession_Unprotected_Multiple_SessionObserverAcceptAndReject) {
  using namespace protocol_handler;
  - ON_CALL(protocol_handler_settings_mock, enable_protocol_5())
  - .WillByDefault(Return(true));
  + ON_CALL(protocol_handler_settings_mock, max_supported_protocol_version())
  + .WillByDefault(Return(PROTOCOL_VERSION_5));
  const size_t maximum_payload_size = 1000;
  InitProtocolHandlerImpl(0u, 0u, false, 0u, 0u, 0, maximum_payload_size);
  