2. Remove restriction on mathml deprecated features (pr epub-specs#2137)

See github pull request epub-specs#2137.

See github issue epub-specs#2118.

Matt Garrish: duga had opened this initially, and we went back and forth.
… i think his concern is that we're imposing a restriction on MathML's deprecated features that MathML itself doesn't impose on its features.
… they are deprecated, but you're still allowed to have them in MathML.
… we are imposing on their spec.
… he's asked that we not restrict these deprecated features.
… I agree, and I don't think us putting a ban on MathML features moves the needle on MathML rendering.
… math rendering is made for web browsers, and we just use what they do.
… if getting rid of these restrictions smooths over a spec weirdness, that is fine.
… people don't hand write MathML anyway.
… so this PR takes out the MUST NOT that was there before.

Dave Cramer: this is in keeping with changes we've been making incrementally over the last 5-10 years.
… initially we were intervening with the goal of making it easier for people who were trying to write RS from scratch.
… to allow them to skip support for complicated features.
… this PR is in keeping with our take on epub as a container for these other technologies.
… we leave it up to these other specs.

Matt Garrish: I don't think these restrictions work anymore, but they were important at the time.

Dave Cramer: so I would propose we merge this.

Murata Makoto: +1.

Proposed resolution: merge PR 2137, close 2118. (Dave Cramer)

Dave Cramer: +1.

Shinya Takami (高見真也): +1.

Masakazu Kitahara: +1.

Matt Garrish: +1.

Toshiaki Koike: +1.

Matthew Chan: +1.

Resolution #2: merge PR 2137, close issue 2118.