Remove design_management_flag feature flag from codebase

In https://gitlab.com/gitlab-org/gitlab-ee/issues/12600 the feature was tested in production and enabled by default.

This issue represents the task of:

• removing the design_management_flag from the codebase,
• removing via /chatops (if not already)

A changelog entry does not need to be added, as the feature has been enabled by default and has already been announced.