Fix a python 2.6.5 compatibility

The upcoming python 2.6.5 release has a change that makes delattr(obj,
attr) fail for slots-enabled objects if the attr is not already set.

To prevent against this, we only run the delattr if the attribute is
already set.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>