Remove preserve_markdown feature flag

Remove the preserve_markdown feature flag and associated code, following the unfortunately tumultuous journey it had with blockers requiring somewhat uncapped R&D to solve (sourcepos fidelity), and lack of a clear value proposition.

See conversation at #474407 (comment 2958672057).

Related Issues

• Rollout issue: #474407
• Parent epic: &7722

Tasks

• Create MR to remove the preserve_markdown feature flag:
  • Remove all references to the feature flag from the codebase
  • Remove the YAML definitions for the feature from the repository
• Clean up the feature flag from all environments by running: /chatops run feature delete preserve_markdown --dev --pre --ops --staging --staging-ref --production
• Close [FF] `preserve_markdown` -- Allows preserving u... (#474407)
• Close Preserve markdown using the Rust based backend ... (&7722)