google: Don't offer a PasswordBased interface

This was a temporary measure back when Google did not support OAuth2
for CalDAV. Now that they do, we can drop this.

In any case, the workaround didn't work with accounts using 2-factor
authentication. This will make those people happy.

This reverts 89c33547

See: https://bugzilla.gnome.org/show_bug.cgi?id=686804
     https://bugzilla.gnome.org/show_bug.cgi?id=688364