Do no longer set the 'ref' field readonly

Because it is set to readonly conditionally on the view.
Setting the field to readonly prevents users to import it with csv files.

base_partner_sequence/partner.py

@@ -21,7 +21,7 @@
 #
 ##############################################################################
 
-from openerp.osv import orm, fields
+from openerp.osv import orm
 
 
 class ResPartner(orm.Model):
@@ -65,10 +65,6 @@ class ResPartner(orm.Model):
                              context=context)
         return vals.get('is_company') or not vals.get('parent_id')
 
-    _columns = {
-        'ref': fields.char('Reference', size=64, readonly=True),
-    }
-
     def _commercial_fields(self, cr, uid, context=None):
         """
         Make the partner reference a field that is propagated