Drop call to QApplication::desktop in x11locker

We previously needed to create the root window so that we would be
notified when the root window changed size.

Since then API has moved to QScreen to get window sizes which has other
ways of monitoring changes.

This is for Qt6 porting.

Testing done:
- removed this whole block of code and confirmed it still does
something
 - removed just this one line and confirmed input is still grabbed