[selectors-4] Remove issue

Author: fantasai

selectors/Overview.bs

@@ -900,13 +900,6 @@ Internal Structure</h4>
 	so some of these currently-invalid selectors (e.g. ''::first-line :any-link'')
 	may become valid in the future.
 
-	Issue: This defines the sibling (and in impossible cases, descendant combinators) to be invalid
-	when applied after pseudo-elements in a selector.
-	We could alternately make them valid but not matching anything--
-	but there's a good chance we'll later expand the featureset to allow such relationships in more cases
-	(e.g. ''::first-line > :any-link''),
-	and this handling follows our standard practice of "make unsupported features syntactically invalid".
-
 	The children of such <a>pseudo-elements</a> can simultaneously be children of other elements, too,
 	turning the <a>selector match list</a> into a directed acyclic graph.
 	However, at least in CSS, their rendering must be defined so as to maintain the tree-ness of the <a>box tree</a>.