Revert "Fix segfault with missing screens"

This reverts commit aba0ad12.

The reason for revertion is that it leaks pixmaps.
QX11Info::display should not return a nullptr if there is no QScreen.
This needs fixing in Qt, not workarounds in our software. None of our
X11 specific code in plasma-workspace or frameworks can handle the case
that the Display* or xcb_connection_t* becomes null suddenly. Neither
can Qt internally. If it would happen Qt would abort.

The only application in our workspace which would be "somewhat" safe
is KWin because it caches the returned Display after first invokation
to QX11Info::display.