cmd/compile: remove unused (dead) fields

Change-Id: I4816a788e7b89b76dc70d05f4b176e99684d0680
Reviewed-on: https://go-review.googlesource.com/18830
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -932,13 +932,9 @@ func isfrog(c int) bool {
 }
 
 type yySymType struct {
-	yys  int
-	node *Node
-	list *NodeList
-	typ  *Type
-	sym  *Sym
-	val  Val
-	op   Op
+	sym *Sym
+	val Val
+	op  Op
 }
 
 const (