[FIX] base: Remove wrong comment

comment stating date of currency rates
As all rates where moved in demo data

Closes #10017

openerp/addons/base/res/res_currency_data.xml

 <?xml version="1.0" encoding="utf-8"?>
 <openerp>
     <data noupdate="1">
-        <!-- The Following currency rates are considered as on 1st Jan,2010 against EUR. -->
         <!-- Currencies -->
         <record id="USD" model="res.currency">
             <field name="name">USD</field>