Draft: Remove the enable_auto_assign_gitlab_duo_pro_seats NamespaceSetting column ignore

What does this MR do and why?

❗ Do not merge this MR before %18.5 in order to respect the database column dropping process ❗

As the final step of Remove the namespace setting enable_auto_assign... (#555041), this MR removes the ignore_column for the NamespaceSetting enable_auto_assign_gitlab_duo_pro_seats column.

This column was ignored in Ignore the enable_auto_assign_gitlab_duo_pro_se... (!197454 - merged) and dropped in Draft: Drop the auto_assign_gitlab_duo_pro_seat... (!196673).

References

Relevant issue: Remove the namespace setting enable_auto_assign... (#555041).

Part of the initiative of https://gitlab.com/gitlab-org/gitlab/-/issues/548750+.

Screenshots or screen recordings

No visual changes expected.

How to set up and validate locally

CI pipeline should continue to pass.

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