Drop support for nscd

Drop support for nscd

nscd is known to be racy [1] and it was already deprecated and later dropped in
Fedora a while back [1,2]. We don't need to support obsolete stuff in systemd,
and the cache in systemd-resolved provides a better solution anyway.

We announced the plan to drop nscd in d44934f3785ad9ca4aab757beb80a9b11ba4bc04.

[1] https://fedoraproject.org/wiki/Changes/DeprecateNSCD
[2] https://fedoraproject.org/wiki/Changes/RemoveNSCD

The option is kept as a stub without any effect to make the transition easier.