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 <[email protected]>

Author: clsung

oauth2_test.go

@@ -28,19 +28,6 @@ func (t *mockTransport) RoundTrip(req *http.Request) (resp *http.Response, err e
 	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",