Remove circuits.CircuitDag (#5733)

This class has moved to contrib.

Removed CircuitDag and Unique from the old location
and have removed them from the global namespace.
Users can still access cirq.contrib.CircuitDag.

Author: dstrain115

cirq-core/cirq/__init__.py

@@ -65,7 +65,6 @@
     AbstractCircuit,
     Alignment,
     Circuit,
-    CircuitDag,
     CircuitOperation,
     FrozenCircuit,
     InsertStrategy,
@@ -74,7 +73,6 @@
     PointOptimizer,
     QasmOutput,
     TextDiagramDrawer,
-    Unique,
 )
 
 from cirq.devices import (

cirq-core/cirq/circuits/__init__.py

@@ -19,7 +19,6 @@
 from cirq.circuits.qasm_output import QasmOutput
 
 from cirq.circuits.circuit import AbstractCircuit, Alignment, Circuit
-from cirq.circuits.circuit_dag import CircuitDag, Unique
 from cirq.circuits.circuit_operation import CircuitOperation
 from cirq.circuits.frozen_circuit import FrozenCircuit
 from cirq.circuits.insert_strategy import InsertStrategy