[IMP] fields: remove unnecessary code

In `__getattr__`, remove the test on accessing `_attrs`.  This situation should
never happen, since the attribute `_attrs` is set in method `__init__()`.

openerp/fields.py

@@ -350,8 +350,6 @@ class Field(object):
 
     def __getattr__(self, name):
         """ Access non-slot field attribute. """
-        if name == '_attrs':
-            raise AttributeError(name)
         try:
             return self._attrs[name]
         except KeyError: