Retire gitlab_enable_admin_mode_sign_in

What does this MR do and why?

Follow up of Speed up admin feature specs (!139400 - merged)

Replace gitlab_enable_admin_mode_sign_in helper for enable_admin_mode!.

This improves the developer experience: It creates a single function to access the admin area, with a parameter to switch between a fast or a slow mode.

I suggest to be reviewed in this fashion:

1. First commit: remove the gitlab_enable_admin_mode_sign_in helper and modify enable_admin_mode!.
2. Second commit: deal with the special cases.
3. Third commit: automatic replacement of all the rest of cases.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

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