The MathJax fallback was inadvertently broken during recent
refactoring of the <markdown> component and has long been untested.

This removes the fallback for the moment and in doing so
provides a clearer definition of what math we support -
the subset supported by KaTeX.

Looking at our public annotations, there is very little use
of complex math currently.

When we find a clear use case for it, we can revisit supporting
additional math beyond what KaTeX supports.

See https://trello.com/c/9KkjsO6v/66-remove-the-mathjax-fallback
for more background.