Remove use_ids_for_markdown_upload_urls flag
What does this MR do and why?
The feature flag has been enabled on GitLab.com for a week so we can remove this
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
Related to #461757 (closed)
Edited by Heinrich Lee Yu