KDED: respond to output being completely removed

Summary:
On Wayland outputs are not merely disconnected, they are completely removed from
the config. In order to apply config that matches the new configuration of available
outputs we need to listen to output removal, not just change in the connected
property.

Reviewers: sebas, davidedmundson

Reviewed By: sebas

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9903