DAG to circuit converter can use _append, should not copy registers. (#3176)
DAG to circuit converter can use _append, should not copy registers. (#3176)

* Use QuantumCircuit._append in dag_to_circuit converter.

* Don't copy registers in dag_to_circuit converter.

QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py