Remove 'category' ignore rule from 'service_access_tokens'

What does this MR do and why?

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

• [%16.6]: In Gitlab Rails: Consolidate SaaS mechanism for is... (#426950 - closed) we added the ignore rule for the category column for service_access_tokens table. Here the link to the MR.
• [%16.7]: In #428784 (closed) and the related MR we are removing the column.
• [%16.8]: we should remove the ignore rules introduced by #426950 (closed)

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How to set up and validate locally

This is no-op change

Related to #428785 (closed)