dirmngr: Implement command KS_DEL for ldap servers.

* dirmngr/ks-action.c (ks_action_del): Change some variable names and
comments.
* dirmngr/ks-engine-ldap.c (ks_ldap_del): Implement.

Tested using OpenLDAP on Linux and with an LDS and AD on Windows.

• GnuPG-bug-id: T5447