core: do not GC units that have FDs stored (#41435)

If a unit has FileDescriptorStorePreserve=yes we'll keep its FDs around
in case it starts again. But if there are no reverse dependencies
referencing it, we'll also GC it and lose all the FDs, which defeats the
point of the setting (which is opt-in).

Do not GC units that have FDs stored to avoid this.

Follow-up for b9c1883a9cd9b5126fe648f3e198143dc19a222d