Remove deprecated Bit.index and Bit.register usage.

[kdk]

Summary

Address deprecation warnings raised by Qiskit/qiskit#6069 .

Details and comments

[kdk]

Currently failing state tomography tests:

test.tomography.test_process_tomography.TestProcessTomographyCVX.test_bell_2_qubits
-----------------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/Users/kevin.krsulichibm.com/q/qiskit-ignis/test/tomography/test_process_tomography.py", line 51, in test_bell_2_qubits
    choi, choi_ideal = run_circuit_and_tomography(bell, q2, self.method)

      File "/Users/kevin.krsulichibm.com/q/qiskit-ignis/test/tomography/test_process_tomography.py", line 36, in run_circuit_and_tomography
    choi = tomo_fit.fit(method=method).data

      File "/Users/kevin.krsulichibm.com/q/qiskit-ignis/qiskit/ignis/verification/tomography/fitters/process_fitter.py", line 150, in fit
    trace_preserving=True, **kwargs))

      File "/Users/kevin.krsulichibm.com/q/qiskit-ignis/qiskit/ignis/verification/tomography/fitters/cvx_fit.py", line 207, in cvx_fit
    prob.solve(**kwargs)

      File "/Users/kevin.krsulichibm.com/q/qiskit-ignis/.tox/py37/lib/python3.7/site-packages/cvxpy/problems/problem.py", line 396, in solve
    return solve_func(self, *args, **kwargs)

      File "/Users/kevin.krsulichibm.com/q/qiskit-ignis/.tox/py37/lib/python3.7/site-packages/cvxpy/problems/problem.py", line 754, in _solve
    self.unpack_results(solution, solving_chain, inverse_data)

      File "/Users/kevin.krsulichibm.com/q/qiskit-ignis/.tox/py37/lib/python3.7/site-packages/cvxpy/problems/problem.py", line 1068, in unpack_results
    "Try another solver, or solve with verbose=True for more "

    cvxpy.error.SolverError: Solver 'SCS' failed. Try another solver, or solve with verbose=True for more information.

[mtreinish]

The CI failures here are because of the aer release (and the wheels taking a little bit of time to upload since they need to be compiled). These changes look good and pass docs and test jobs (where aer had a precompiled binary) so I'm going to merge this as is to unblock the release pr #568 if there issues on macOS or windows I can address them there.