Refactor registration methods

What does this MR do and why?

Related Upstream MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/112533

The MR we submitted to Upstream has completed the preparation for refactor,

and the purpose of current MR is to remove duplicate methods and templates.

• Remove useless resource method: https://jihulab.com/gitlab-cn/gitlab/-/issues/2579
• Refactor signup_box_template method: https://jihulab.com/gitlab-cn/gitlab/-/issues/2580

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/cc EM @mtan-gitlab-cn