services: elogind: Rely on D-Bus activation.

Previously elogind was explicitly spawned by the Shepherd.  Now it is
activated by dbus-daemon on demand.

* gnu/packages/freedesktop.scm (elogind)[arguments]: Add
'fix-service-file' phase.
* gnu/services/desktop.scm (elogind-shepherd-service): Remove.
(elogind-dbus-service): New procedure.
(elogind-service-type): Do not extend SHEPHERD-ROOT-SERVICE-TYPE.  Use
'elogind-dbus-service' for DBUS-ROOT-SERVICE-TYPE.