[BUG] Delete repo do not delete related issue_label entrys

[6543]

tested with gitea 1.14.0+dev-877-g0e5c6c449

reproduce:

1. create repo with 1 issue and 1 label
2. assign label to issue
3. delete repo
4. exec: select COUNT(issue_label.id) from issue_label LEFT JOIN issue ON issue_label.issue_id = issue.id LEFT JOIN label ON issue_label.label_id = label.id WHERE issue.id IS NULL OR label.id IS NULL;