Remove the `export_designs` feature flag

What

Remove the :export_designs feature flag. This was added in !14702 (merged) and !14700 (merged).

Owners

• Team: groupknowledge
• Most appropriate slack channel to reach out to: #g_create_knowledge
• Best individual to reach out to: @.luke

Expectations

### What are we expecting to happen?

The feature flag is enabled by default, so we don't expect any issues.

Roll Out Steps

• [-] Enable on staging
• [-] Test on staging
• [-] Ensure that documentation has been updated
• [-] Enable on GitLab.com for individual groups/projects listed above and verify behaviour
• [-] Announce on the issue an estimated time this will be enabled on GitLab.com
• [-] Enable on GitLab.com by running chatops command in #production
• [-] Cross post chatops slack command to #support_gitlab-com and in your team channel
• [-] Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel