Don't send CI usage email notifications for self-hosted instances

What does this MR do?

It fixes a bug where we were sending CI usage email notifications for customers running their own instance of GitLab EE.

REF: #12998 (closed)

Does this MR meet the acceptance criteria?

Conformity

Performance and testing

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

  • Label as security and @ mention @gitlab-com/gl-security/appsec
  • The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
  • Security reports checked/validated by a reviewer from the AppSec team