Prep v231.2: login/elogind.c: Remove bus_forward_agent_released()

This method is called from a systemd manager that is the system
instance to inform all user instances of systemd about the pending
cgroup release.

elogind on the other hand is always there just once. And the release
of cgroups is handled by the local cgroups manager, which should be
provided by the running init system.

Even if there is no cgroup management, so elogind sets itself up as
a small cgroups manager itself, there aren't any user instances that
could react on the forwarding anyway.