Don't create a guest login if user went to /login

This fixes an unintuitive behaviour where, if you follow a link to
riot.im/app/#/login, we take you to the login page, but not before we've
registered a guest account (or restarted the MatrixClient with the stored
creds).

This actually ends up simplifying some of the startup dance, as we special-case
the registration flows earlier on.

src/Lifecycle.js

@@ -35,9 +35,6 @@ import { _t } from './languageHandler';
  * Called at startup, to attempt to build a logged-in Matrix session. It tries
  * a number of things:
  *
- * 0. if it looks like we are in the middle of a registration process, it does
- *    nothing.
- *
  * 1. if we have a loginToken in the (real) query params, it uses that to log
  *    in.
  *
@@ -80,14 +77,6 @@ export function loadSession(opts) {
     const guestIsUrl = opts.guestIsUrl;
     const defaultDeviceDisplayName = opts.defaultDeviceDisplayName;
 
-    if (fragmentQueryParams.client_secret && fragmentQueryParams.sid) {
-        // this happens during email validation: the email contains a link to the
-        // IS, which in turn redirects back to vector. We let MatrixChat create a
-        // Registration component which completes the next stage of registration.
-        console.log("Not registering as guest: registration already in progress.");
-        return q();
-    }
-
     if (!guestHsUrl) {
         console.warn("Cannot enable guest access: can't determine HS URL to use");
         enableGuest = false;