oauth2: remove mockCache since NewTransportFromTokenStore() removed.

Since NewTransportFromTokenStore() is removed, mockCache is therefore
useless. It should be safe to remove it, too.

Change-Id: I5678684af31e5de75e420b28d859e33909e0a718
Reviewed-on: https://go-review.googlesource.com/22113
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
diff --git a/oauth2_test.go b/oauth2_test.go
index 448673b..a9aecd6 100644
--- a/oauth2_test.go
+++ b/oauth2_test.go
@@ -28,19 +28,6 @@
 	return t.rt(req)
 }
 
-type mockCache struct {
-	token   *Token
-	readErr error
-}
-
-func (c *mockCache) ReadToken() (*Token, error) {
-	return c.token, c.readErr
-}
-
-func (c *mockCache) WriteToken(*Token) {
-	// do nothing
-}
-
 func newConf(url string) *Config {
 	return &Config{
 		ClientID:     "CLIENT_ID",