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](https://gitlab.com/gitlab-org/gitlab/-/blob/4b339309c754e8e69dd66466f37a3d11c7a66461/app/mailers/emails/in_product_marketing.rb#L15-15) and everything that calls it. Services, Workers, etc
**note**
:warning: be sure we do NOT turn off the following when we remove our code: :warning:
* [build_ios_app_guide_email](https://gitlab.com/gitlab-org/gitlab/-/blob/4b339309c754e8e69dd66466f37a3d11c7a66461/app/mailers/emails/in_product_marketing.rb#L24-24)
* [account_validation_email](https://gitlab.com/gitlab-org/gitlab/-/blob/4b339309c754e8e69dd66466f37a3d11c7a66461/ee/app/mailers/ee/emails/in_product_marketing.rb#L6-6)
issue