Moved core work items backend work out of work_item feature flag

What does this MR do and why?

Everything that is currently in the :work_items feature flag has been defaulted to on for the previous two milestones. Instead of removing the feature flag, we are going to keep it in and move other parts of work items that are ready to be "defaulted on" over to it.

This MR removes the backend checks we had for the feature flag.

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