nixosTests.ldap: remove

This seems to have worked in 15f105d4 (5
months ago) but broke somewhere in the meantime.

The current module doesn't seem to be underdocumented and might need a
serious refactor. It requires quite some hacks to get it to work (see
https://github.com/NixOS/nixpkgs/issues/86305#issuecomment-621129942),
or how the ldap.nix test used systemd.services.openldap.preStart and
made quite some assumptions on internals.

Mic92 agreed on being added as a maintainer for the module, as he uses
it a lot and can possibly fix eventual breakages. For the most basic
startup breakages, the remaining openldap.nix test might suffice.