Drop css3-layout.vim

This module was renamed to “CSS Template Layout Module” and published as
Working Group Note.

See also: http://www.w3.org/TR/2015/NOTE-css-template-3-20150326/

test/test.html

@@ -300,11 +300,6 @@
 }
 </style>
 <style>
-.layout::slot(b) {
-  display: same;
-}
-</style>
-<style>
 .line-grid {
   line-grid: create;
   line-snap: block-start;