MDL-79487 oauth2: do not delete refresh token

when `log_out` is called from `\core\oauth2\client` it will delete the refresh token,
what it actually needs to use it to get a new access token
actually logging out is not needed here, the only thing we need to make sure is,
the invalid access token is removed from the session
that is done by storing `null`

lib/oauthlib.php

@@ -449,7 +449,7 @@ abstract class oauth2_client extends curl {
     public function is_logged_in() {
         // Has the token expired?
         if (isset($this->accesstoken->expires) && time() >= $this->accesstoken->expires) {
-            $this->log_out();
+            $this->store_token(null);
             return false;
         }