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).