adtrust: remove nttrustpartner parameter

MS-ADTS spec requires that TrustPartner field should be equal to the
commonName (cn) of the trust. We used it a bit wrongly to express
trust relationship between parent and child domains. In fact, we
have parent-child relationship recorded in the DN (child domains
are part of the parent domain's container).

Remove the argument that was never used externally but only supplied by
trust-specific code inside the IPA framework.

Part of https://fedorahosted.org/freeipa/ticket/5354

Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>

ipaserver/plugins/trust.py

@@ -1477,10 +1477,6 @@ class trustdomain(LDAPObject):
             cli_name='sid',
             label=_('Domain Security Identifier'),
         ),
-        Str('ipanttrustpartner?',
-            label=_('Trusted domain partner'),
-            flags=['no_display', 'no_option'],
-        ),
     )
 
     # LDAPObject.get_dn() only passes all but last element of keys and no kwargs