Remove placeholder work item description templates

What does this MR do and why?

Removes the client side placeholder data and moves the feature behind a temporary work_items_description_templates feature flag rather than work_items_alpha.

References

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• BE: Work Items - Create/edit flows: Apply template (#501097 - closed)
• FE: Work Items - Create/edit flows: Apply template (#428690 - closed)
• Work items: Description templates (&16088 - closed)

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Related to #512208 (closed)