[FIX] purchase: remove incorrect PO-based stock move valuation for incoming shipments

[FIX] purchase: remove incorrect PO-based stock move valuation for incoming shipments

As discussed on bug 923191, the current override
is incorrect because it will silently replace the cost
of incoming stock moves with PO prices for products w/
standard costing - this should only concern products
with average costing for which no cost was manually
provided.
And in fact we can completely remove this override
because even for products with average costing,
the PO prices are already provided as default cost
prices in the picking/move reception wizard, so
the PO price is already taken into account and the
cost will never be missing.
Thanks to Graeme Gellatly for the excellent
analysis!

lp bug: https://launchpad.net/bugs/923191 fixed

bzr revid: odo@openerp.com-20120201113724-y18fna1v7z6kxka4