units: make enablement of s-n-wait-online.service follow systemd-networkd.service (#5635)

In 58a6dd15 s-n-wait-online.service was added
to presets to synchronize the presets with the state after installation. But it
is harmful to have s-n-wait-online.service enabled when s-n.service is
disabled, because s-n-wait-online.service has Requsite=s-n.service and cannot
be activated. Thus remove s-n-wait-online.service from presets again, and let
it be enabled whenever s-n.service is enabled.

During installation we create enablement symlinks by hand, and since s-n.service
is enabled, s-n-w-o.service should be enabled too, so the symlink should still
be created during installation.

https://bugzilla.redhat.com/show_bug.cgi?id=1433459#c15(cherry picked from commit 9e496560)