Remove Gemnasium service

What does this MR do?

It removes all code related to the Gemnasium service.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/6010

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [ ] Tests added for this feature/bug
• Conforms to the code review guidelines
• [ ] Conforms to the [merge request performance guidelines](https://docs.gitlab.com/ee/development/merge_request_performance_guidelines.html)
• [ ] Conforms to the style guides
• [ ] Conforms to the database guides