Finish the rest of the changes from the geometry positioning

Summary:
Commit d7c230ab191654986b8bb2fd9bf47d48805abf40 changes the window to
fill the screen.

However it left dead code in shutdown dialog which was also
repositioning it - and worse...positioning it in a different place
which is just a mess.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2619