Move account validation email under EE

What does this MR do and why?

This MR is part of #346233 (closed).

It moves the mailer action for Account Validation email (from app/mailers/emails/in_product_marketing.rb) and Gitlab::Email::Message::AccountValidation (lib/gitlab/email/message/account_validation.rb) under EE since the email is EE-specific.

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