Remove api.SVGStylable

This was a mixin-like interface in SVG 1.1:
https://www.w3.org/TR/SVG11/types.html#InterfaceSVGStylable

However, it was never exposed as an interface in itself, and this entry
without any subfeatures doesn't map to anything real for web developers.

MDN cleanup: mdn/content#5140

Part of mdn#7752.