files: Keep single sorted list of fds

After fds restore set merge we no longer need two lists.

Don't BUG in eventpoll when the tfd is not on the list.
This situation means that it's restored, so it's safe
to report OK from this place.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>