disable use of mathml in online HTML docs

In ~~#2442~~ enabled MathJax output in additon to mathml since mathml support is being phased out in browsers. Right now version of tex4ht have started to produce incorrect MathML for some ET docs eg GRHydro’s use of \array

\begin{equation}
  \begin{array}{lcl}
    % tex4ht fails unless \mathbf is in a group
    \Delta_{\mathrm{upw}} & = & {\mathbf{q}}_{i} - {\mathbf{q}}_{i-1}\\
    \Delta_{\mathrm{loc}} & = & {\mathbf{q}}_{i+1} - {\mathbf{q}}_{i}\,,\\
  \end{array}
\end{equation}

make tex4ht output invalid mathml:

‌

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