`layer()` is invalid CSS and should not be supported

[bartveneman]

Was triggered by @romainmenke in csstools/postcss-plugins#1525

Remove the code + tests that support this invalid case.

[bartveneman]

CodePen to demonstrate that FF/Chrome doesn't actually apply the CSS when using layer() https://codepen.io/bartveneman/pen/wBwBjdX?editors=1100

[bartveneman]

fixed in b1da449