Remove 'format' flag and most markdown dependencies.

[kallentu]

Relevant work for #3535.

Part 1 of removing markdown in Dartdoc. I added TODOs for the parts I'll include next CL, I prefer doing this in smaller chunks (for example, made a todo in RendererFactory because I'll do the larger removal later.).

• Removed the format option.
• For behaviour that relied on that flag result, defaulted to html.
• Removed markdown specific tests.
• Removed format option tests.

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[srawlins]

Excellent! Looks great