ads: Remove redundant occurrences of "virtual"

As suggested by clang-tidy's "modernize-use-override" check and to match
Qt Creator's coding guidelines.

Change-Id: I2cfef113e21a0cd44d18a5b98e9bc8427563c53a
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/libs/advanceddockingsystem/dockcontainerwidget.h

@@ -84,7 +84,7 @@ protected:
     /**
      * Handles activation events to update zOrderIndex
      */
-    virtual bool event(QEvent *event) override;
+    bool event(QEvent *event) override;
 
 public: // TODO temporary
     /**
@@ -177,7 +177,7 @@ public:
     /**
      * Virtual Destructor
      */
-    virtual ~DockContainerWidget() override;
+    ~DockContainerWidget() override;
 
     /**
      * Adds dockwidget into the given area.