[kcms] Remove window manager component chooser

In a Wayland world it doesn't make much sense to offer a different
WM anymore. But also on X11 it becomes more and more questionable to
allow changing the WM. E.g. also tooltips need KWin now.

The provided wms were also questionable with Compiz and Metacity being
basically dead and openbox the only working one.

If someone still wants to change WM it's still possibly my modifying the
KDEWM environment envariable.

This change was approved by the shared Plasma and VDG meeting at the
CERN sprint.