cgroup: simplify how instantiated units are mapped to cgroups

cgroup: simplify how instantiated units are mapped to cgroups

Previously for an instantiated unit foo@bar.service we created a cgroup
foo@.service/foo@bar.service, in order to place all instances of the
same template inside the same subtree. As we now implicitly add all
instantiated units into one per-template slice we don't need this
complexity anymore, and instance units can map directly to the cgroups
of their full name.