[css-borders-4] Remove the second value from the corner-*-shape varia…

…nt (#11651)

* [css-borders-4] Remove the corner-*-shape variant

This variant makes sense for border-radius but not for corner-shape,
as corner-shapes have only one value.

Closes #11650

* Remove the /

css-borders-4/Overview.bs

@@ -448,7 +448,7 @@ Corner Shaping: the 'corner-shape' and 'corner-*-shape' properties</h4>
 	<pre class=propdef>
 		Name: corner-top-shape, corner-right-shape, corner-bottom-shape, corner-left-shape,
 			corner-block-start-shape, corner-block-end-shape, corner-inline-start-shape, corner-inline-end-shape
-		Value: <<corner-shape-value>> [ / <<corner-shape-value>> ]
+		Value: <<corner-shape-value>>
 		Initial: round
 		Applies to: all elements where 'border-radius' can apply
 		Inherited: no