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

models/issue_user.go

@@ -69,7 +69,7 @@ func updateIssueUserByAssignee(e Engine, issue *Issue) (err error) {
 // UpdateIssueUserByAssignee updates issue-user relation for assignee.
 func UpdateIssueUserByAssignee(issue *Issue) (err error) {
 	sess := x.NewSession()
-	defer sessionRelease(sess)
+	defer sess.Close()
 	if err = sess.Begin(); err != nil {
 		return err
 	}