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

This reverts commit 68f1dd5ddf.

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

plugin/transfer/notify.go

@@ -1,58 +0,0 @@
-package transfer
-
-import (
-	"fmt"
-
-	"github.com/coredns/coredns/plugin/pkg/rcode"
-
-	"github.com/miekg/dns"
-)
-
-// Notify will send notifies to all configured to hosts IP addresses. If the zone isn't known
-// to t an error will be returned. The string zone must be lowercased.
-func (t *Transfer) Notify(zone string) error {
-	if t == nil { // t might be nil, mostly expected in tests, so intercept and to a noop in that case
-		return nil
-	}
-
-	m := new(dns.Msg)
-	m.SetNotify(zone)
-	c := new(dns.Client)
-
-	x := longestMatch(t.xfrs, zone)
-	if x == nil {
-		return fmt.Errorf("no such zone registred in the transfer plugin: %s", zone)
-	}
-
-	var err1 error
-	for _, t := range x.to {
-		if t == "*" {
-			continue
-		}
-		if err := sendNotify(c, m, t); err != nil {
-			err1 = err
-		}
-	}
-	log.Debugf("Sent notifies for zone %q to %v", zone, x.to)
-	return err1 // this only captures the last error
-}
-
-func sendNotify(c *dns.Client, m *dns.Msg, s string) error {
-	var err error
-
-	code := dns.RcodeServerFailure
-	for i := 0; i < 3; i++ {
-		ret, _, err := c.Exchange(m, s)
-		if err != nil {
-			continue
-		}
-		code = ret.Rcode
-		if code == dns.RcodeSuccess {
-			return nil
-		}
-	}
-	if err != nil {
-		return fmt.Errorf("notify for zone %q was not accepted by %q: %q", m.Question[0].Name, s, err)
-	}
-	return fmt.Errorf("notify for zone %q was not accepted by %q: rcode was %q", m.Question[0].Name, s, rcode.ToString(code))
-}