encoding/xml: remove duplicate test of element presence

Change-Id: If0d9ff107fc6bbdf0231cd48abc23a44816bfe77
Reviewed-on: https://go-review.googlesource.com/85755Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/xml/marshal_test.go

@@ -583,16 +583,6 @@ var marshalTests = []struct {
 		ExpectXML: `<PresenceTest></PresenceTest>`,
 	},
 
-	// A pointer to struct{} may be used to test for an element's presence.
-	{
-		Value:     &PresenceTest{new(struct{})},
-		ExpectXML: `<PresenceTest><Exists></Exists></PresenceTest>`,
-	},
-	{
-		Value:     &PresenceTest{},
-		ExpectXML: `<PresenceTest></PresenceTest>`,
-	},
-
 	// A []byte field is only nil if the element was not found.
 	{
 		Value:         &Data{},