Remove users_helper.rb and related spec

What does this MR do and why?

Related issue: #2914 (closed)

Remove users_helper.rb to allow the code to pass lint temporarily.

Failed job link

https://jihulab.com/gitlab-cn/gitlab/-/jobs/7520021

Changes from

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/118841 https://gitlab.com/gitlab-org/gitlab/-/merge_requests/118390

How to fix

Remove useless methods

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.