oauth2.go: use a more straightforward return value

Change-Id: I72d94836c93dabe50fe67ddb762389f674ba6490
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/653215
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

oauth2.go

@@ -288,7 +288,7 @@ func (tf *tokenRefresher) Token() (*Token, error) {
 	if tf.refreshToken != tk.RefreshToken {
 		tf.refreshToken = tk.RefreshToken
 	}
-	return tk, err
+	return tk, nil
 }
 
 // reuseTokenSource is a TokenSource that holds a single token in memory