Remove the issue_type_uses_work_item_types_table feature flag

What does this MR do and why?

Remove the issue_type_uses_work_item_types_table feature flag

All the proper indexes have been created in .com and the feature has been enabled globally in .com for 2 days now. I has been enabled in staging for a lot longer than that.

This change is not visible to users. The feature flag was introduced to aid in the rollout of #410210 (closed)

MR acceptance checklist

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Related to #386603 (closed)