Remove MailGun Product Onboarding Emails
Summary
Remove in product emails that have converted to Iterable
Context from @dstull
We need to be careful in turning this off as some other non-iterable-destined code has crept into this area. See discussion on that email https://gitlab.com/gitlab-org/gitlab/-/issues/396777#note_1360965397, so we'll need to either move it away from this area in the code a bit first, or somehow work around it so we don't accidentally turn it off too. /cc @eugielimpin
Plan
Remove in_product_marketing_email and everything that calls it. Services, Workers, etc
note