icmp: fix ExtendedEchoRequest extension object

RFC8335 [1] stats: "When applied to the ICMP
   Extended Echo Request message, the ICMP Extension Structure MUST
   contain exactly one instance of the Interface Identification Object."

The InterfaceIdent by name object seems to be a copy/paste issue.

[1]: https://www.rfc-editor.org/rfc/rfc8335.txt

icmp/multipart_test.go

@@ -232,11 +232,6 @@ func TestMarshalAndParseMultipartMessage(t *testing.T) {
 							Type:  2,
 							Index: 911,
 						},
-						&icmp.InterfaceIdent{
-							Class: 3,
-							Type:  1,
-							Name:  "en101",
-						},
 					},
 				},
 			},