fix bug forget to remove Stopwatch when remove repository (#4933)

fix bug forget to remove Stopwatch when remove repository

models/repo.go

@@ -1852,6 +1852,9 @@ func DeleteRepository(doer *User, uid, repoID int64) error {
 		if _, err = sess.In("issue_id", issueIDs).Delete(&IssueWatch{}); err != nil {
 			return err
 		}
+		if _, err = sess.In("issue_id", issueIDs).Delete(&Stopwatch{}); err != nil {
+			return err
+		}
 
 		attachments := make([]*Attachment, 0, 5)
 		if err = sess.