swh.lister.gitlab: Remove TODO about the 403 response code
Multiple issues wish for the api to converge on 429 but nothing is
clear nor documented yet:
- https://gitlab.com/gitlab-com/infrastructure/issues/348
- https://gitlab.com/gitlab-org/gitlab-ce/issues/41309
- https://gitlab.com/gitlab-org/gitlab-ce/issues/46522
The only response code mentioned in the documentation is
403 (https://docs.gitlab.com/ee/api/README.html#status-codes).