[IMP] account: accrual never change account on the fly

Before this commit, the account on the original line was replaced by the
accrual account. This could be problematic for hashed journals. Instead
of having 2 different behaviors for hashed/non hashed journals, we
decided to always create 2 journal entries, which is also more academic:
* We don't replace something on an entry that has been posted
* We don't end up with taxes on the accrual account
* Accountants are used to do it this way

closes odoo/odoo#42787

Signed-off-by: Cedric Snauwaert (csn) <csn@openerp.com>