test: fix return.go to remove unused labels

The gc compiler only gives an error about an unused label if
it has not given any errors in an earlier pass.  Remove all
unused labels in this test because they don't test anything
useful and they cause gccgo to give unexpected errors.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12580044

test/return.go

@@ -272,7 +272,6 @@ func _() int {
 } // ERROR "missing return"
 
 func _() int {
-L:
 	print(1)
 	if x == nil {
 		panic(2)
@@ -972,7 +971,6 @@ func _() int {
 } // ERROR "missing return"
 
 func _() int {
-L:
 	if x == nil {
 		panic(2)
 	} else if x == 1 {
@@ -1666,7 +1664,6 @@ var _ = func() int {
 } // ERROR "missing return"
 
 var _ = func() int {
-L:
 	print(1)
 	if x == nil {
 		panic(2)
@@ -2366,7 +2363,6 @@ var _ = func() int {
 } // ERROR "missing return"
 
 var _ = func() int {
-L:
 	if x == nil {
 		panic(2)
 	} else if x == 1 {