Remove circular dependency on Redactable in migration
Gitlab::BackgroundMigration::RedactLinks
was prepending
EE::Gitlab::BackgroundMigration::RedactLinks
, while
EE::Gitlab::BackgroundMigration::RedactLinks
was trying to include
Gitlab::BackgroundMigration::RedactLinks::Redactable
. Ruby 2.5.3 failed
with an uninitialized constant
(https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/118388511).