Remove use of .device in cirq-google. (quantumlib#4820)

Next step in quantumlib#4744 .  Removes use of device attachment ability in cirq-google. This is a breaking change in the sense that some methods become "more forgiving" which I've highlighted below. Happy to add the tag if people think it's appropriate to label it.

Author: MichaelBroughton

Diff for: cirq/circuits/circuit.py

@@ -67,6 +67,8 @@
 CIRCUIT_TYPE = TypeVar('CIRCUIT_TYPE', bound='AbstractCircuit')
 INT_TYPE = Union[int, np.integer]
 
+_DEVICE_DEP_MESSAGE = 'Attaching devices to circuits will no longer be supported.'
+
 
 class Alignment(enum.Enum):
     # Stop when left ends are lined up.