fix(client): The FxDesktop broker no longer sends the `session_status` message on startup.

The call to `session_status` was originally used to determine which page the content server should start at. Firefox now opens content server with the correct page, so this call is no longer necessary.

`session_status` can cause problems. After a logout from Sync, the content server redirects to /signin and Firefox no longer expects a `session_status` message. A channel timeout occurs, and the user is immediately redirected to /500.html.

fixes #1883
issue #1830
I think we can also rip this out of `app-start.js` and the base broker.