Delete Labels & IssueLabels on Repo Delete too (#15039)

* Doctor: find IssueLabels without existing label

* on Repo Delete: delete labels & issue_labels too

* performance nits

* Add Migration: Delete orphaned IssueLabels

* Migration v174: use Sync2

* USE sess !!!

* better func name

* code format & comment

* RAW SQL

* Update models/migrations/v176.go

* next try?

models/migrations/v174.go

@@ -5,6 +5,8 @@
 package migrations
 
 import (
+	"fmt"
+
 	"xorm.io/xorm"
 )
 
@@ -19,5 +21,15 @@ func addRepoTransfer(x *xorm.Engine) error {
 		UpdatedUnix int64 `xorm:"INDEX NOT NULL updated"`
 	}
 
-	return x.Sync(new(RepoTransfer))
+	sess := x.NewSession()
+	defer sess.Close()
+	if err := sess.Begin(); err != nil {
+		return err
+	}
+
+	if err := sess.Sync2(new(RepoTransfer)); err != nil {
+		return fmt.Errorf("Sync2: %v", err)
+	}
+
+	return sess.Commit()
 }