v206 porting: remove user-session@.service.

From now on, we won't abuse `systemd --user`.
v206 porting: remove session-creating units.

Everything that is connected to setting up a X session (X server, WMs
and session-wide environment variables) is now to be configured via a
DM.
v206 porting: remove Xmodmap and xrdb units.

This is too X-centric and too common. Any decent DM does this via its
Xsession script.
v206 porting: fix graphic units to be session-centric, not Xserver-centric.

The idea is following: the DM will start and stop a dedicated
graphic@.target unit via its Xsession script.
Any graphic units are now bound to that unit.