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

@@ -932,10 +932,6 @@ func isfrog(c int) bool {
 }
 
 type yySymType struct {
-	yys  int
-	node *Node
-	list *NodeList
-	typ  *Type
 	sym *Sym
 	val Val
 	op  Op