Remove outputs handling alternative

Summary:
With f013a436, cd6b69a4, d960be4b and edb200f6 all possible
backends of a Wayland session have been ported to using the internal functions
of AbstractWaylandOutput's for managing outputs.

This removes the alternative code path used before in these backends, what
simplifies the code and is also a prerequisite to removing the Screens global
in the future.

Reviewers: #kwin, apol, zzag

Reviewed By: #kwin, apol, zzag

Subscribers: apol, kwin

Tags: #kwin

Maniphest Tasks: T11459, T11098

Differential Revision: https://phabricator.kde.org/D23485