Remove `design_management_require_hashed_storage` feature flag

What does this MR do?

This MR removes the design_management_require_hashed_storage feature flag.

This feature flag was added in %12.5 and was always default enabled, in order to allow any on-prem customers to disable the requirement if they ran into difficulty with the new requirement for hashed storage #13428 (comment 238729038).

We can now safely remove it #35477 (comment 310589036).

• Issue: #35477 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry Feature flag has been enabled by default since !19259 (merged)
• [-] Documentation (if required) Documentation added in !19259 (merged)
• 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