Drop statusTip from MainWindow

This was introduced by c24cbf29 back in 2007, but:
1. It violates the current HIG [1] about the status bar.
2. It causes an ugly visual bug when quickly hovering
   with the mouse on the entries in the Open Recent submenu.
3. Its text is almost useless. I don't even know what it meant with 'click and hold'.

[1]: https://techbase.kde.org/Projects/Usability/HIG/StatusBar