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/pkg/dnsutil/dedup.go

@@ -1,12 +0,0 @@
-package dnsutil
-
-import "github.com/miekg/dns"
-
-// Dedup de-duplicates a message.
-func Dedup(m *dns.Msg) *dns.Msg {
-	// TODO(miek): expensive!
-	m.Answer = dns.Dedup(m.Answer, nil)
-	m.Ns = dns.Dedup(m.Ns, nil)
-	m.Extra = dns.Dedup(m.Extra, nil)
-	return m
-}