Drop compositing suspension dbus api

The main motivation behind this change is to encapsulate X11 compositor
implementation to the point that it can be moved from libkwin to
kwin_x11.

There are two options: either get rid of suspend and resume requests, or
add suspend and resume virtual functions in the Compositor. The apps
must definitely not rely on the dbus api, they must use the X11 api.
Compositing suspension api works only on X11 and it's quite obscure,
it's not used by any other KDE component.

So this patch goes for the most simplest option: dropping the requests.