Support for markdown files would only typeset text as bold or italic
when it appeared inside the corresponding inline Markdown syntax. It
didn't render the Markdown, but rather approximated what the text might
look like if rendered. Because this provides little real value, and
because HLJS sometimes incorrectly parses these Markdown constructs,
markdown is dropped from the highlighting layer.
Bug: Issue 9056
Change-Id: I97865dc8b522dca5a50264456274590fac83dae9
db8691a96b