Prevent unnecessary sessions in API requests

What does this MR do and why?

Avoid setting unnecessary session values when doing the admin_mode? check. This also prevents creation of session records for API requests when admin mode is enabled.

Related to #437145 (closed)

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How to set up and validate locally

1. Run gdk redis-cli monitor | grep "session:gitlab" to see sessions being saved to Redis
2. Enable admin mode: https://docs.gitlab.com/ee/administration/settings/sign_in_restrictions.html#enable-admin-mode-for-your-instance
3. Create a personal access token for an admin user
4. Run curl http://localhost:3000/api/v4/version --header "PRIVATE-TOKEN: <ADMIN_TOKEN>"

Before this change, you will see something like:

"setex" "session:gitlab:2::f08271e97a86989c0a6e399aa7fed48b72a48a646a1b6d2f1d2b832a197a5735" "604800" "\x04\b{\x06I\"\x16current_user_mode\x06:\x06EF{\x00"

With this change, there would be no session being saved for API requests.

This also happens with any other API request. This is due to the Gitlab::Auth::CurrentUserMode.new(user).admin_mode? check which ends up calling current_session_data[ADMIN_MODE_START_TIME_KEY] to check the value in the session.