Refactor session close as xorm already does everything needed internally (#2020)

models/webhook.go

@@ -267,7 +267,7 @@ func UpdateWebhook(w *Webhook) error {
 // ID must be specified and do not assign unnecessary fields.
 func deleteWebhook(bean *Webhook) (err error) {
 	sess := x.NewSession()
-	defer sessionRelease(sess)
+	defer sess.Close()
 	if err = sess.Begin(); err != nil {
 		return err
 	}