With f013a4369c2e, cd6b69a4d2db, d960be4b3f39 and edb200f6bcd5 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.