Delete the correct item in removeDesktop

Summary:
QList::erase will move the data underneath where the iterator is
pointing, we want to delete the entry we're about to remove from the
list.

Test Plan: Relevant tests now behave better

Reviewers: #kwin, broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D16885