Remove unused remark markdown deserializer

What does this MR do and why?

Remove remark markdown deserializer

Remove last bit of unused code related to preserve_unchanged_markdown feature.

Related to Remove code behind `preserve_unchanged_markdown` (#439783 - closed)

Previous MRs related to this effort:

• Remove code behind preserve_unchanged_markdown (!158759 - merged)
• Remove wysiwyg/prosemirror glfm specs (!160563 - merged)

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