Remove unused in product email metrics items

What does this MR do and why?

There is related code to in product marketing emails that we didn't remove, context here: #427204 (comment 1588651267). This MR removes in_product_marketing_emails_service and two related instrumentation classes whose metrics were removed previously in !131860 (merged).

A follow-up MR will remove app/models/users/in_product_marketing_email.rb.

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 #427341 (closed)