Open the zone, double click the NS record, then click the delete button to
remove the incorrect NS record.
Honestly, it probably should have removed the NS record when the zone was
removed from the DNS server in Active Directory, but it's not a big deal it
didn't remove the NS record. That is unless the zone isn't AD integrated and
that server had a secondary zone on it. In which case if the server still
has a zone in it and it is secondary, you should leave the NS record if you
are going to leave the secondary zone in that server. If not delete the zone
and the NS record.