[16.10] Remove ai_access_token ignore rule

What does this MR do and why?

Step 3 of dropping the column.
In %16.9, we dropped the column: !134770 (merged).
This MR cleans up the ignore rule from the model.

See https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-columns

MR acceptance checklist

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How to set up and validate locally

This is no-op change.

Related to #424113 (closed)