Disable the partial HTML and MathML rendering options for Math extension.

MathML mode was so incomplete most people thought it simply didn't work (bug 25646).
HTML modes often rendered poorly (eg req bug 24207 to set default to PNG on some sites)

This may cause regressions in that simple "equations" of just variables that came out as HTML
will now render as PNGs that don't get aligned properly with the text baseline. Fixing this is
covered by bug 32694: https://bugzilla.wikimedia.org/show_bug.cgi?id=32694 to retrieve the
baseline info from dvipng and position images to fit.

Note that because of the way user options are pulled in to the oarser cache key, some folks
may see cached pages with their old settings until they get redone or they save their prefs
again and have it normalized.