Remove deprecated class SerializableDevice (#5743)

Remove deprecated property GridDevice.qubits.
Clean up documentation references to the SerializableDevice and
update the circuit validation example in the specification.md doc.

Part of #5050

cirq-google/cirq_google/__init__.py

@@ -59,7 +59,6 @@
     GoogleNoiseProperties,
     GridDevice,
     NoiseModelFromGoogleNoiseProperties,
-    SerializableDevice,
     Sycamore,
     Sycamore23,
 )

cirq-google/cirq_google/devices/__init__.py

@@ -20,5 +20,3 @@
 from cirq_google.devices.known_devices import Sycamore, Sycamore23
 
 from cirq_google.devices.grid_device import GridDevice
-
-from cirq_google.devices.serializable_device import SerializableDevice

cirq-google/cirq_google/devices/grid_device.py

@@ -308,15 +308,6 @@ def metadata(self) -> cirq.GridDeviceMetadata:
         """Get metadata information for the device."""
         return self._metadata
 
-    # Some user code using SerializableDevices gets the qubit list via `device.qubits`.
-    # This is a stopgap solution to prevent user breakage with the change to GridDevice.
-    @property  # type: ignore
-    @cirq._compat.deprecated(
-        deadline='v0.16', fix='Change `device.qubits` to `device.metadata.qubit_set`.'
-    )
-    def qubits(self) -> List[cirq.Qid]:
-        return sorted(self._metadata.qubit_set)
-
     def validate_operation(self, operation: cirq.Operation) -> None:
         """Raises an exception if an operation is not valid.
 

cirq-google/cirq_google/devices/grid_device_test.py

@@ -490,11 +490,3 @@ def test_to_proto_empty():
     assert len(device.metadata.qubit_pairs) == 0
     assert device.metadata.gateset == cirq.Gateset()
     assert device.metadata.gate_durations is None
-
-
-def test_grid_device_qubits():
-    device_info, spec = _create_device_spec_with_horizontal_couplings()
-    device = cirq_google.GridDevice.from_proto(spec)
-
-    with cirq.testing.assert_deprecated('device.qubits', deadline='v0.16'):
-        assert device.qubits == device_info.grid_qubits