Remove create_group_level_work_items feature flag

What does this MR do and why?

While working on Dynamic workItemTypes list based on namespace (#524828), I noticed that we don't need the create_group_level_work_items feature flag anymore. This feature flag was controlling if we could create epic WorkItems on the group level, but the same now can be achieved with WorkItems::Type.allowed_group_level_types.

References

• Add work_item_epics feature flag (!157443 - merged)
• Dynamic workItemTypes list based on namespace (#524828)

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