Remove all GUI service launching to ~/.xsessionrc

All graphical services (ie. Guake, Nitrogen, Light-Locker) are launched
from ~/.xsessionrc instead of with systemd services or from the WM
directly.

Systemd user service sessions are singular for each user but a user can
start multiple (graphical) login sessions so strictly graphical
services (ones which have a visible effect on the display) need to be
launched multiply, once for each graphical session.

~/.xsessionrc is a better, generic way of launching display services on
startup.