fix(diagnostic_graph_aggregator): remove unused message alias (autowarefoundation#8722)

feat(diagnostic_graph_aggregator): remove unused message alias

Author: isamu-takagi

system/diagnostic_graph_aggregator/test/src/test2.cpp

@@ -29,7 +29,6 @@ using namespace diagnostic_graph_aggregator;  // NOLINT(build/namespaces)
 
 using diagnostic_msgs::msg::DiagnosticArray;
 using diagnostic_msgs::msg::DiagnosticStatus;
-using tier4_system_msgs::msg::DiagnosticGraph;
 
 constexpr auto OK = DiagnosticStatus::OK;
 constexpr auto WARN = DiagnosticStatus::WARN;