The <mfenced> element [1] is not part of MathML Core [2] or MathML AAM [3] and was removed in bug 1603773. It should be treated as an unknown MathML element, that is like an `<mrow>`. This commit performs some follow-up code cleanup: * Remove a11y mapping, instead map it like an mrow. * Remove obsolete deprecation message. * Don't include mfenced in the sanitizer's default element allow list. The spec [4] does not mention an explicit list. * Preference mathml.mfenced_element.disabled was removed, so no need to force it when running tests. * Tweak some code comments. [1] https://developer.mozilla.org/en-US/docs/Web/MathML/Element/mfenced [2] https://w3c.github.io/mathml-core/#mathml-elements-and-attributes [3] https://w3c.github.io/mathml-aam/ [4] https://wicg.github.io/sanitizer-api Differential Revision: https://phabricator.services.mozilla.com/D157580
