Remove MailGun onboarding email controller

What does this MR do and why?

Remove in product onboarding email controller. Remove in_product_marketing_emails_enabled setting.

Groups::EmailCampaignsController was used previously before switching to Iterable for onboarding emails. Right now it only has random request once in couple days from old emails and we are OK with that:

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#	MR
1	Remove MailGun product onboarding email (!130318 - merged)
2	this MR
3	Remove MailGun onboarding messages (!130626 - merged)
4	Remove in_product_marketing_email modal call in... (!131129 - merged)

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Related to #418137 (closed)