Remove the store_designs_in_lfs feature flag

What does this MR do?

This MR removes the store_designs_in_lfs feature flag from the codebase.

Issue https://gitlab.com/gitlab-org/gitlab-ee/issues/12158.

The feature flag was added in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13389.

This MR does not include a changelog entry, because the feature has been enabled by default since https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13389 and that MR contained the changelog.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance 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