[non-SVG outputs] revisit role=math

We are currently adding role=math to the output parent. However, this leads to poor results in the current AT landscape, cf. the test results at http://codepen.io/pkra/pen/zqrwQx and also https://github.com/w3c/aria/issues/290.

It seems better to remove role=math (since it breaks labels where present). If there's no suitable information in the source, we might want to consider adding a generic aria-label.

(For comparison, for the SVG output, we had changed the role to img https://github.com/mathjax/MathJax/issues/1192.)