When `spec/lib/gitlab/mail_room/mail_room_spec.rb` is run before
`spec/lib/system_check/incoming_email/imap_authentication_check_spec.rb`,
`Gitlab::MailRoom.enabled_configs` leaks state from one test to another.

To fix this, we should clean up the instance variable after the tests finish.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/332706