[css-overflow-3] Remove issue about -moz-hidden-unscrollable because WG feels that floats projecting invisible geometry outside the box is weird, and Mozilla didn't seem to have a strong opinion on it. Can be reopened if necessary.

Author: fantasai

css-overflow-3/Overview.bs

@@ -340,11 +340,6 @@ Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</
 			through any mechanism,
 			and therefore the box is not a <a>scroll container</a>.
 
-			Issue: Mozilla implements -moz-hidden-unscrollable,
-			which is similar to ''clip'',
-			except that it does not cause the element to establish a BFC.
-			Should we match that?
-
 		<dt><dfn>scroll</dfn>
 		<dd>
 			This value indicates that