Remove premature login cancel so post-login popup gets shown (Bug 105052...

This takes out the `oncancel()` call from login.js which tests for a dismissed window. We miight want to take out the whole `popup_interval`. This might be a problem, but I have heard the whole popup is going away in favour of a solution that is embedded in the page, which would be better in a variety of ways.
but I couldn't find an event listener for `login_cancel`. maybe my awking isn't working.