Firefox 13: remove work around for mtable@width in native MathML ouput mode

[fred-wang]

Firefox 13 is still in the aurora channel, but when it is released the work around for mtable@width will not be necessary any more, since it will have support for that attribute (*). This will make the following tests pass:

LaTeXToMathML/environments/multiline-1a.html
LaTeXToMathML/environments/multiline-1b.html
LaTeXToMathML/layout/shove-1.html

(*) See https://bugzilla.mozilla.org/show_bug.cgi?id=722880

[fred-wang]

Firefox 13 has been released. I've prepared 32bfe5a for this issue and verified that the three LaTeXToMathML tests now pass.

[dpvc]

Thanks, Fred, your changes look good.