Remove MD renderer factory and related markdown renderers.

[kallentu]

As title says. Work for #3535.

There's an additional arc of work where I want to just make everything default to HTML renderers and renaming everything and removing the extra interfaces that aren't needed now that the markdown renderers are removed.

That's coming after this though. (I prefer to have smaller segmented CLs.)

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[srawlins]

So much red!