go/ast: Walk: do not walk comment list

A comment to that effect was introduced
with rev d332f4b9cef5 but the respective
code wasn't deleted.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6304086

src/pkg/go/ast/walk.go

@@ -344,9 +344,6 @@ func Walk(v Visitor, node Node) {
 		}
 		Walk(v, n.Name)
 		walkDeclList(v, n.Decls)
-		for _, g := range n.Comments {
-			Walk(v, g)
-		}
 		// don't walk n.Comments - they have been
 		// visited already through the individual
 		// nodes