Revert automatic Duo seat assignment changes behind stale feature flag

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/548750+.

This MR rolls back changes from the partially implemented feature in Iteration 1.0, Automatic Duo pro assignment via... (&13637).

In particular, the following MRs are reverted:

• Allow group owners to enable duo pro auto assign (!151511 - merged)
• Auto assign the new members a duo pro seat (!152058 - merged)

This MR also removes the auto_assign_gitlab_duo_pro_seats feature flag added in Add enable_auto_assign_gitlab_duo_pro_seats nam... (!150824 - merged).

Note: documentation in !152058 (merged) was already rolled back in Removing Duo auto assign reference from docs (!159008 - merged).

A follow-up MR Draft: Drop the auto_assign_gitlab_duo_pro_seat... (!196673) removes the enable_auto_assign_gitlab_duo_pro_seats namespace setting to toggle the removed feature.

References

Relevant issue: https://gitlab.com/gitlab-org/gitlab/-/issues/548750+

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start GDK in SaaS mode (GITLAB_SIMULATE_SAAS=1).
2. Log into any user and navigate to a top-level group. Note its group ID. (Create one if needed.)
3. In the Rails console, enable the auto_assign_gitlab_duo_pro_seats for that group: Feature.enable(:auto_assign_gitlab_duo_pro_seats, Group.find(<group_id>)).
4. Go to Settings > General > Permissions and group features (/groups/:group_name/-/edit#js-permissions-settings)
5. Verify that the Automatic assignment of GitLab Duo Pro seats no longer exists.

MR acceptance checklist

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