Fix the deprecation warning for ClassicallyControlledOperation

[yjt98765]

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

Cirq/cirq-core/cirq/protocols/json_serialization.py

Lines 281 to 288 in 01ae51e

	if 'cirq_type' in base_dict:
	# TODO: upgrade to ValueError in v0.15
	warnings.warn(
	f"Found 'cirq_type': '{base_dict['cirq_type']}' in _json_dict_. "
	f"Custom values of this field are not permitted, and will produce "
	"an error starting in Cirq v0.15.",
	DeprecationWarning,
	)

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

Cirq/docs/dev/serialization.md

Lines 91 to 95 in 01ae51e

	1. The object should have a `_json_dict_` method that returns a dictionary
	containing a `"cirq_type"` key as well as keys for each of the value's
	attributes. If these keys do not match the names of the class' initializer
	arguments, a `_from_json_dict_` class method must also be defined.
	Typically the `"cirq_type"` will be the name of your class.

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

close #4763

[vtomole]

I think this is left over from #4704. @yjt98765 Are there any other cirq_type keys left besides this one?

[yjt98765]

Are there any other cirq_type keys left besides this one?

No. Based on code searching results, I think all the cirq_type in _json_dict_ methods have been removed in #4704. ClassicallyControlledOperation is added after #4704. In addition, there are no more such warnings when running json_serialization_test.py.

[vtomole]

LGTM