core/service: drop unneeded unit_add_to_gc_queue()

core/service: drop unneeded unit_add_to_gc_queue()

Follow-up for a1d315730ffddf283d4bb9d73878fbcd97a4d244
and 6ac62d61db737b01ad3776a7688d8a4c57b3f7d9

With the aforementioned commits, unit_release_resources()
is dispatched in a dedicated queue, and Service.n_keep_fd_store
has been dropped, hence the comment is outdated. Moreover,
the unit is added to GC queue in unit_notify() already.
No other unit types do this in corresponding _enter_dead()
functions, nor does Service need it anymore.