cmd/compile/internal/gc: remove unused field Type.Siggen

Remove unused field Type.Siggen in go.go.

This change passes go build -toolexec 'toolstash -cmp' -a std.

Change-Id: Ia61fe65a226c913fbf4a11a71d7453c56aa46c0e
Reviewed-on: https://go-review.googlesource.com/14372
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

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

@@ -160,7 +160,6 @@ type Type struct {
 	Trecur      uint8 // to detect loops
 	Printed     uint8
 	Embedded    uint8 // TFIELD embedded type
-	Siggen      uint8
 	Funarg      uint8 // on TSTRUCT and TFIELD
 	Copyany     uint8
 	Local       bool // created in this file

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

@@ -3556,7 +3556,6 @@ func copytype(n *Node, t *Type) {
 	if n.Name != nil {
 		t.Vargen = n.Name.Vargen
 	}
-	t.Siggen = 0
 	t.Method = nil
 	t.Xmethod = nil
 	t.Nod = nil