Get rid of the user_namespace API helper method

What does this MR do?

This gets rid of the user_namespace API helper which is used only once and doesn't have an explicit behavior (it does a permission check with an early return so it should end with a !; it memoizes its return value in an instance variable which is never explicitely used).

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/20728.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

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