Revert ESCAPED_TT tokenization (wikitext, caa69cc)

I don't want to introduce special escapes into the grammar, even though this one works. In the interests of consistency the correct solution is to just embed literal backticks in a <nowiki></nowiki> span, as demonstrated in the specs. Literal backticks are already passed through unaltered in <pre></pre> and <tt>spans</tt> anyway, which is arguably the most common case.

Signed-off-by: Greg Hurrell <greg@hurrell.net>