Handling of the "math" script tag

[fred-wang]

This is the current text at https://mathml-refresh.github.io/mathml-core/#opentype-features:

        <p>
          Good mathematical rendering requires use of non-Unicode glyphs.
          Mathematical fonts may only provide these glyphs when the
          <code>math</code> script tag is enabled [[OPEN-FONT-FORMAT]],
          so user agents must
          ensure that it is the case when rendering text inside the
          <code>&lt;mtext&gt;</code> element.
        </p>

From https://docs.microsoft.com/en-us/typography/opentype/spec/math:

         'math' | Script tag to be used for features in math layout. The only language  system supported with this tag is the default language system.

I believe this was only needed because of rtlm ( w3c/mathml#148 ) and ssty ( #135 ). But we moved them to level-2 so it seems the same should be true for this.

[fred-wang]

consensus from 2020/06/23: move to level-2

[fred-wang]

text removed.
Incidentally, the text was saying "mtext" but it should really be all MathML token elements (minus mspace)