runtime: Remove now unnecessary pad field from ParFor.

It is not needed due to the removal of the ctx field.
    
    Reviewed-on: https://go-review.googlesource.com/16525

From-SVN: r229616

libgo/go/runtime/runtime_unix_test.go

@@ -42,7 +42,7 @@ func TestGoroutineProfile(t *testing.T) {
 	if testing.Short() {
 		max = 100
 	}
-	stk := make([]runtime.StackRecord, 100)
+	stk := make([]runtime.StackRecord, 128)
 	for n := 0; n < max; n++ {
 		_, ok := runtime.GoroutineProfile(stk)
 		if !ok {