Remove preserve_markdown FF and supporting code

What does this MR do and why?

Removes the preserve_markdown feature flag, all supporting code, and dependencies used only by it (and a few more dependencies that were once, but no longer, used by it).

References

• Remove `preserve_markdown` feature flag and ass... (#584484 - closed)
• [FF] `preserve_markdown` -- Allows preserving u... (#474407 - closed)
• Preserve markdown using the Rust based backend ... (&7722 - closed)
• The discussion where we resolved to remove it: #474407 (comment 2958672057)

How to set up and validate locally

1. Check out the branch; run yarn to ensure removed packages are removed.
2. Consider restarting your GDK for good measure.
3. Try using the rich text editor in any Markdown field. It should continue to work exactly as before, and the saved Markdown (and rendered result) should look the same as it did when using the editor before.

MR acceptance checklist

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