Remove unused in product email items

There is related code to in product marketing emails that we didn't remove. We should clean it up.

Context in this thread

Plan

Remove the following related code:

!138221 (merged):

• app/services/namespaces/in_product_marketing_emails_service.rb and code around it including `
• lib/gitlab/usage/metrics/instrumentations/in_product_marketing_email_cta_clicked_metric.rb and lib/gitlab/usage/metrics/instrumentations/in_product_marketing_email_sent_metric.rb
  • validate the process for removing metrics in docs or with the groupproduct analytics team.
  • Docs for removing metrics: https://docs.gitlab.com/ee/development/internal_analytics/metrics/metrics_lifecycle.html#remove-a-metric

!138835 (merged):

• - app/models/users/in_product_marketing_email.rb
  • This will also include dropping a table in the database. Follow proper process for that.