[FIX] point_of_sale: close session without ordered

We cannot close a session without POS order as we are giving explicitly
a name to the account_move, it is not possible to delete it at the end
when it is empty.

So we don't name it anymore, and it takes its name from the journal
sequence when it is posted, which actually do exactly the same as
before, except that it avoid to have gaps in the sequence and allow
deletion of the move if it's not named.

closes odoo/odoo#37787

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>