[FIX] point_of_sale: apply fiscal position

Commit https://github.com/odoo/odoo/commit/5afb5ecd04cf49be6bedb655b187b19a65e1e02d#diff-1c14accbe350f7a177b3e81e5846861bL1363
removed the call to `_map_tax_fiscal_position`. Fiscal positions are not
applied anymore...

Instead of applying the fiscal position in the `compute_all` method as
it used to be, we apply it just before the call. This mimics the
behavior of SO/PO/Invoices, where the taxes are changed directly on the
order line.