Follow-up from "Use billing email to create subscriptions"

The following discussion from !47105 (merged) should be addressed:

• @nicolasdular started a discussion: (+1 comment)

  Non-blocking for this MR We might consider not sending along the billing_email in the API, since it is part of the customer on customerDot anyway. We also need to find a way for gl.com users to provide their billing email in GitLab if it should be different.

Scope of the issue

This Issue takes care of the first part of the suggestion (not sending along the billing_email``) while leaving the second one (find a way for gl.com users to provide their billing email in GitLab) for later discussion. Given the undergoing work to move subscriptions management to GitLab, building a way to provide the billing address might not be the case at this point.