[Breaking Change] Remove support for CSS custom property transformation

[pmdartus]

Early on we added support for CSS custom property transformation to the LWC style compiler. This was introduced to emulate CSS custom properties on IE11 and Safari 9. With the deprecation of compat, we should also drop support for CSS custom properties transformations, since it widely complicates the LWC style compiler code and more specifically the serialization. Prior to this, all the CSS transformations for synthetic shadow emulation could be done in place.

[nolanlawson]

This is done