[css-overflow] Remove border-box overflow per WG resolution

Author: fantasai

css-overflow/Overview.bs

@@ -337,37 +337,6 @@ Introduction</h2>
 		in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
 	</p>
 
-<h3 id="border-box-overflow">Border box overflow</h3>
-
-	<p class="issue">
-		This concept has been proposed for some uses, such as for
-		determining what the 'outline' property goes around, and
-		as the basis of a coordinate system for specifying clips and masks,
-		but it's not clear if it's needed.
-	</p>
-
-	<p>
-		The <dfn>border-box overflow</dfn> of a box is the
-		union of the box's border edge and the border edges of
-		the box's descendants.</p>
-	</p>
-
-	<p class="issue">
-		If needed, define more formally, as for scrollable overflow above.
-		(Maybe even share the definitions in an appropriate way!)
-	</p>
-
-	<p>
-		The <dfn>border-box overflow region</dfn> is the non-rectangular region
-		occupied by the <a>border-box overflow</a>, and the
-		<dfn>border-box overflow rectangle</dfn> is
-		the minimal rectangle whose axis is aligned to the box's axes
-		and contains the <a>border-box overflow region</a>.
-		Note that the <a>border-box overflow rectangle</a> is a rectangle
-		in the box's coordinate system, but might be non-rectangular
-		in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
-	</p>
-
 <h2 id="overflow-properties">Overflow properties</h2>
 
 	<p>