Remove compat data for mfrac@bevelled (#17735)
Remove compat data for mfrac@bevelled (#17735)

Remove compatibility data for the `bevelled` attribute of the `mfrac`
element. It has only been implemented in Firefox and removed from
Firefox 71 (december 2019) so considered irrelevant per [1].

[1] https://github.com/mdn/browser-compat-data/blob/main/docs/data-guidelines/index.md#removal-of-irrelevant-features
chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,mdn/browser-compat-data