Revert "Implement notifies for transfer plugin (#3972)" (#3995)

This reverts commit 68f1dd5ddf.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/etcd/xfr.go

@@ -1,17 +1,25 @@
 package etcd
 
 import (
+	"context"
 	"time"
 
 	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
 )
 
-// Serial returns the serial number to use.
+// Serial implements the Transferer interface.
 func (e *Etcd) Serial(state request.Request) uint32 {
 	return uint32(time.Now().Unix())
 }
 
-// MinTTL returns the minimal TTL.
+// MinTTL implements the Transferer interface.
 func (e *Etcd) MinTTL(state request.Request) uint32 {
 	return 30
 }
+
+// Transfer implements the Transferer interface.
+func (e *Etcd) Transfer(ctx context.Context, state request.Request) (int, error) {
+	return dns.RcodeServerFailure, nil
+}