Minor refactor

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

Author: jacobperron

rcljava/src/main/java/org/ros2/rcljava/action/ActionServerImpl.java

@@ -184,7 +184,7 @@ public synchronized final long getHandle() {
   private final T actionTypeInstance;
   private final String actionName;
   private long handle;
-  private final GoalCallback<? extends GoalRequestDefinition<T>> goalCallback;
+  private final GoalCallback goalCallback;
   private final CancelCallback<T> cancelCallback;
   private final Consumer<ActionServerGoalHandle<T>> acceptedCallback;
 
@@ -329,8 +329,7 @@ private ActionServerGoalHandle<T> executeGoalRequest(
     }
 
     // Call user callback
-    GoalCallback callback = this.goalCallback;
-    GoalCallback.GoalResponse response = callback.handleGoal(requestMessage);
+    GoalCallback.GoalResponse response = this.goalCallback.handleGoal(requestMessage);
 
     boolean accepted = GoalCallback.GoalResponse.ACCEPT == response;
     responseMessage.accept(accepted);