[ksmserver] Drop internal loading of alternative window managers

Summary:
ksmserver's loading of other window manager relies on files in
share/windowmanagers/$name.desktop with the Exec line of the window
manager.

This was something previously installed by plasma-desktop and removed in
2016. All this code path is now redundant as well as the wmChanged DBus
method used by the deleted KCM.

Setting KDEWM manually will still work as before.

Test Plan:
Logged in
Got kwin as before

Reviewers: #plasma, broulik, apol

Reviewed By: #plasma, broulik, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21758