Revert "cgroup.c: check return value of unit_realize_cgroup_now()"

This reverts commit 1f11a0cdfe397cc404d61ee679fc12f58c0a885b.
Do not realloc strings, which are already in the hashmap as keys

This prevents corruption of the hashmap, because we would free() the
keys in the hashmap, if the unit is already in there, with the same
cgroup path.