doc: Replace Q_DECL_OVERRIDE by override in snippets

And remove redundant virtual.

Change-Id: If0650409b88ad962f6713d082d9095675f4c68e8
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/corelib/doc/snippets/code/src_corelib_kernel_qabstractnativeeventfilter.cpp

@@ -52,7 +52,7 @@
 class MyXcbEventFilter : public QAbstractNativeEventFilter
 {
 public:
-    virtual bool nativeEventFilter(const QByteArray &eventType, void *message, long *) Q_DECL_OVERRIDE
+    bool nativeEventFilter(const QByteArray &eventType, void *message, long *) override
     {
         if (eventType == "xcb_generic_event_t") {
             xcb_generic_event_t* ev = static_cast<xcb_generic_event_t *>(message);

src/corelib/doc/snippets/code/src_corelib_kernel_qabstractnativeeventfilter.h

@@ -44,6 +44,6 @@
 class MyCocoaEventFilter : public QAbstractNativeEventFilter
 {
 public:
-    bool nativeEventFilter(const QByteArray &eventType, void *message, long *) Q_DECL_OVERRIDE;
+    bool nativeEventFilter(const QByteArray &eventType, void *message, long *) override;
 };
 //! [0]

src/corelib/doc/snippets/code/src_corelib_thread_qthread.cpp

@@ -55,7 +55,7 @@ class MyObject;
 class WorkerThread : public QThread
 {
     Q_OBJECT
-    void run() Q_DECL_OVERRIDE {
+    void run() override {
         QString result;
         /* ... here is the expensive or blocking operation ... */
         emit resultReady(result);