Remove CMARK_ENGINE used in older markdown rendering

Brett Walkerrequested to merge
545119-remove-cmark_engine-used-in-older-markdown-rendering into master

What does this MR do and why?

We no longer need the old cmark/commonmarker rendering engine, which has been replaced by the gitlab-glfm-markdown gem. So let's remove it.

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Related to #545119 (closed)

Edited by Brett Walker

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