Remove Qt version checks.

Qt 5.3 is the minimum requirement these days. Remove all fallback code
from sources and project files.

Change-Id: If6188a471197acadda4d6baee71804ba1a8026c6
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/debugger/debuggerplugin.cpp

@@ -2699,9 +2699,7 @@ QMessageBox *showMessageBox(int icon, const QString &title,
         title, text, QMessageBox::StandardButtons(buttons),
         ICore::mainWindow());
     mb->setAttribute(Qt::WA_DeleteOnClose);
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 1, 0))
     mb->setTextInteractionFlags(Qt::TextSelectableByMouse);
-#endif
     mb->show();
     return mb;
 }