Remove dnsutil.Dedup (#1867)

Remove the code and remove the call in etcd and kubernetes handlers.
This does mean we should not add dups in the first place, which means
adding maps in backend_lookup to prevent dups from begin added.

This should cut down on the allocations because dnsutil.Dedup is very
expensive by converting everything to strings, we avoid doing that now.

plugin/kubernetes/handler.go

@@ -4,7 +4,6 @@ import (
 	"context"
 
 	"github.com/coredns/coredns/plugin"
-	"github.com/coredns/coredns/plugin/pkg/dnsutil"
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
@@ -79,9 +78,6 @@ func (k Kubernetes) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.M
 	m.Answer = append(m.Answer, records...)
 	m.Extra = append(m.Extra, extra...)
 
-	// TODO(miek): get rid of this by not adding dups in the first place, dnsutil.Append()?
-	m = dnsutil.Dedup(m)
-
 	state.SizeAndDo(m)
 	m, _ = state.Scrub(m)
 	w.WriteMsg(m)