Remove __props__ support check in example code

doc/extending/extending_theano.txt

@@ -291,12 +291,6 @@ Example: Op definition
         __props__ = ()
 
         def make_node(self, x):
-            # check that the theano version has support for __props__.
-            # This next line looks like it has a typo,
-            # but it's actually a way to detect the theano version
-            # is sufficiently recent to support the use of __props__.
-            assert hasattr(self, '_props'), "Your version of theano is too old 
-            to support __props__."
             x = theano.tensor.as_tensor_variable(x)
             # Note: using x_.type() is dangerous, as it copies x's broadcasting 
             # behaviour