Revert "Explicit currency for order prices"

This reverts commit 77474bfe6ca7b2501e5798dcde6faf66c7f20ad9.
Explicit currency for order prices

Related to woothemes#4143

When generating the prices involved in an order user the orders'
currency instead of the shop's default currency. Otherwise, the prices
are misleading when the order currency is different than the shop's
default currency.