core/swap.c: Do not add Before=swap.target to swap units.

core/swap.c: Do not add Before=swap.target to swap units.

The fstab generator adds Before=swap.target by default, and when creating
a custom .swap unit, you can also add Before=swap.target to the unit.

However, it is impossible to not have this ordering dependency right now.
Virtually all existing setups likely use the fstab generator, so this
change is unlikely to break anything.