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

⚠ be sure we do NOT turn off the following when we remove our code: ⚠

• build_ios_app_guide_email
• account_validation_email