Update box-3.vim

https://www.w3.org/TR/2020/WD-css-box-3-20200421/

This WD removes all new properties and values.

after/syntax/css/box-3.vim

@@ -1,9 +0,0 @@
-syn match cssBoxProp contained "\<margin-trim\>"
-syn match cssBoxAttr contained "\<in-flow\>"
-" These will be moved to block-3
-syn match cssBoxProp contained "\<clear-after\>"
-syn match cssBoxProp contained "\<float-displace\>"
-syn match cssBoxProp contained "\<indent-edge-reset\>"
-syn keyword cssBoxAttr contained scrollbar panner marquee decendants
-syn match cssBoxAttr contained "\<block-within-page\>"
-syn match cssBoxAttr contained "\<\(margin\|border\|padding\|content\)-edge\>"