Fix the deprecation warning for ClassicallyControlledOperation (qua…

…ntumlib#4780)

The `_json_dict_` method of `ClassicallyControlledOperation` contains a `cirq_type` key, which causes a deprecation warning:

https://github.com/quantumlib/Cirq/blob/01ae51eebf3b18a5cbee9fc0c697d4e1511c07f2/cirq-core/cirq/protocols/json_serialization.py#L281-L288

It is easy to fix by removing this item. Nevertheless, the real problem is in the manual for serialization, which says:

https://github.com/quantumlib/Cirq/blob/01ae51eebf3b18a5cbee9fc0c697d4e1511c07f2/docs/dev/serialization.md?plain=1#L91-L95

It says a `cirq_type` is required, which contradicts the code. To avoid confusing developers in the future, I have modified it.

close quantumlib#4763

cirq/ops/classically_controlled_operation.py

@@ -171,7 +171,6 @@ def _circuit_diagram_info_(
 
     def _json_dict_(self) -> Dict[str, Any]:
         return {
-            'cirq_type': self.__class__.__name__,
             'conditions': self._conditions,
             'sub_operation': self._sub_operation,
         }