Remove create_epic_form feature flag

What does this MR do?

Remove references to :create_epic_form feature flag.

This feature flag was used to ship the feature incrementally, so there is no use case for it after deployment.

Related issue: #230742 (closed)

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done