[clang-tools-extra] [clang-doc] Remove Markdown library implementation (PR #209257)

[Paul Kirth via cfe-commits]

ilovepi wrote:

To be clear there are no users of this code yet, other than tests. The plan was to have this evolve over the summer, but that will no longer happen due to changes in resourcing and availability.

https://github.com/llvm/llvm-project/pull/209257