UIFe: hide the "force fallback" control

[Sebastien Bacher]

Hi,

I would like to ask an ui freeze exception for
https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/825250

Basically the Ubuntu system settings info panel has a "force fallback"
option which doesn't work in Ubuntu because we select the session to use
from the login manager, I suggest that we hide the control from the ui.

There should be no string changes (it's just hidding some widgets), the
documentation team probably need to check how that feature is documented
though

Thanks,
Sebastien Bacher