Do not compute precision: displayed exchange rate is related to selected currency, so there is no point in doning any rounding to the value to display.
Also, unify the way exchange rate is computed.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@22253 20353a03-c40f-0410-a6d1-a30d3c3de9de