remove unused utils.GetByteOrder

This function was needed when we supported both Q039 (using big endian
encoding) and ealier versions (using little endian encoding).

internal/wire/public_header_test.go

@@ -430,11 +430,7 @@ var _ = Describe("Public Header", func() {
 			})
 
 			Context("in big endian", func() {
-				version := protocol.Version39
-
-				BeforeEach(func() {
-					Expect(utils.GetByteOrder(version)).To(Equal(utils.BigEndian))
-				})
+				version := versionBigEndian
 
 				It("writes a header with a 1-byte packet number", func() {
 					b := &bytes.Buffer{}