Remove custom-property-no-outside-root

[Paul-Hebert]

This rule requires that custom properties only be set on the :root element which severely limits how they can be used.

It requires this in order to make the old PostCSS polyfill for custom properties work. That treats custom properties as immutable variables (like SCSS vars) which limits their power. From the docs:

It currently just aims to provide a future-proof way of using a limited subset (to :root selector) of the features provided by native CSS custom properties.

We should be able to use the full set of features provided by CSS custom properties. The polyfill behavior was for IE11 (which didn't support custom props.) On most new projects we no longer support IE11 so we should ditch this rule.

https://github.com/suitcss/stylelint-suitcss/blob/master/rules/custom-property-no-outside-root/README.md