cgroup: Don't explicitly check for member in UNIT_BEFORE

cgroup: Don't explicitly check for member in UNIT_BEFORE

The parent slice is always filtered ahead of time from UNIT_BEFORE, so
checking if the current member is the same as the parent unit will never
pass.

I may also write a SLICE_FOREACH_CHILD macro to remove some more of the
parent slice checks, but this requires a bit of a rework and general
refactoring and may not be worth it, so let's just do this for now.