Remove the access token for the old IS

Pretty important that we don't send that to the new IS...

src/components/views/settings/SetIdServer.js

@@ -137,6 +137,7 @@ export default class SetIdServer extends React.Component {
         let newFormValue = this.state.idServer;
         if (!errStr) {
             MatrixClientPeg.get().setIdentityServerUrl(fullUrl);
+            localStorage.removeItem("mx_is_access_token", fullUrl);
             localStorage.setItem("mx_is_url", fullUrl);
             newFormValue = '';
         }