settings: remove obsolete options
    
    Re-remove obsolete options cache_ttl, psearch, serial_autoincrement
    and zone_refresh. They were already removed in
    c3bfe1a62ac4f8a73207bf4e80d64a4a3a58d9e4, but re-introduced in
    refactoring.
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>