Revert "Try setting common NameUsed prerequisite in dynamic DNS update"

This reverts commit d75bc5be.

Likely unnecessary.

dns_challenge.go

@@ -66,10 +66,8 @@ func (d *dnsValidator) makeMsg(zone string, rrs []dns.RR, remove bool) *dns.Msg
 	m := new(dns.Msg)
 	m.SetUpdate(zone)
 	if remove {
-		m.NameUsed(rrs)
 		m.Remove(rrs)
 	} else {
-		m.NameUsed(rrs)
 		m.RemoveRRset(rrs)
 		m.Insert(rrs)
 	}