Use default __hash__ method for ERP5 objects

If an object has no uid yet and has no acquisition context to the portal,
getUid() fails.

Default is derived from id(aq_base(ob)) which is even more efficient.

The method on XMLObject was also redundant because it inherited the same one
from Base.

(cherry picked from commit 347804af)

product/ERP5Type/Base.py

@@ -2872,10 +2872,6 @@ class Base( CopyContainer,
     return dict(redirect_url=container.absolute_url(),
                 selection_index=None, selection_name=None)
 
-  # Hash method
-  def __hash__(self):
-    return hash(self.getUid())
-
   security.declareProtected(Permissions.ModifyPortalContent, 'setGuid')
   def setGuid(self):
     """

product/ERP5Type/XMLObject.py

@@ -108,9 +108,5 @@ class XMLObject( Folder ):
       conduit = ERP5Conduit()
       conduit.addNode(object=self, xml=xml)
 
-    # Hash method
-    def __hash__(self):
-      return hash(self.getUid())
-
 
 InitializeClass(XMLObject)