Avoid setting an explicit token fetcher for the known flows.
diff --git a/oauth2.go b/oauth2.go
index 24ddc3a..0062f59 100644
--- a/oauth2.go
+++ b/oauth2.go
@@ -66,7 +66,6 @@
 		if err != nil {
 			return err
 		}
-		o.TokenFetcherFunc = makeThreeLeggedFetcher(o)
 		o.AuthURL = au
 		o.TokenURL = tu
 		return nil