Core: Remove windowActivated signal from MainWindow

That makes no sense now that we potentially have multiple windows.
It is also not available outside of the Core plugin and unused in
Core.

Change-Id: I32b491e4ec078892e9ad3a8fb62616e4567549a6
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/coreplugin/mainwindow.cpp

@@ -894,18 +894,6 @@ void MainWindow::removeContextObject(IContext *context)
         updateContextObject(m_activeContext);
 }
 
-void MainWindow::changeEvent(QEvent *e)
-{
-    QMainWindow::changeEvent(e);
-    if (e->type() == QEvent::ActivationChange) {
-        if (isActiveWindow()) {
-            if (debugMainWindow)
-                qDebug() << "main window activated";
-            emit windowActivated();
-        }
-    }
-}
-
 void MainWindow::updateFocusWidget(QWidget *old, QWidget *now)
 {
     Q_UNUSED(old)