[FIX] purchase: revert revision 7677 to set a default unit of measure to 'Unit'
This allows to field a purchase.order without showing the units of measure if not product is selected
Revision 7677 was integrated to fix lp:958897 (no change of uom when selecting a product whose uom is in the category 'Units'). This fix did not solve it properly (only for initial value) and introduced another problem. A better fix will be done on the onchange product.