fix gh-5816. When we unpickle, we don't always had c code, the us regenerate it if needed later

theano/scalar/basic.py

@@ -4025,7 +4025,6 @@ class Composite(ScalarOp):
         self.prepare_node_called = set()
         self.init_fgraph()
         self.init_py_impls()
-        assert self._c_code
 
 
 class Compositef32(object):