core: rework socket/service GC logic

core: rework socket/service GC logic

There's no need to set the no_gc bit for service units that socket units
prepare, as we always keep a proper reference (as maintained by unit_ref_set())
on them, and such references are honoured by the GC logic anyway. Moreover,
explicitly setting the no_gc bit is problematic if the socket gets GC'ed for a
reason, as the service might then leak with the bit set.