Remove MailGun onboarding messages

What does this MR do and why?

Remove in product email messages that have converted to Iterable

#	MR
1	Remove MailGun product onboarding email (!130318 - merged)
2	Remove MailGun onboarding email controller (!130611 - merged)
3	this MR
4	Remove in_product_marketing_email modal call in... (!131129 - merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #418137 (closed)