[css-scrollbars] Drop stale references to "the properties" and IE

This is no longer the design the spec is going after,
so this Note should have been removed when the design changed.

See #1956

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -275,9 +275,6 @@ Value Definitions</h3>
 	Note: when a user interacts with a scrollbar (e.g. hovering or activating),
 	implementations may alter which scrollbar colors apply to which scrollbar parts.
 
-	Note: IE uses named System Colors as defaults for each of the scrollbar color properties.
-	See related <a href="https://github.com/w3c/csswg-drafts/issues/1956">Issue 1956</a>.
-
 	<div class="example">
 		<p>
 		The following example