DYNDNS: remove zone command
Remove zone command from message to nsupsate. This command is generally
used to hint nsupdate. In correctly configured environment such
information should be obtained via DNS.
If DNS does not provide necessary information we give other hints.
For more details see:
https://fedorahosted.org/sssd/wiki/DesignDocs/DDNSMessagesUpdate
Resolves:
https://fedorahosted.org/sssd/ticket/2495
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>