Remove qobj run in AerBackend

[hhorii]

Summary

Remove qobj support in backend.run(), which was deprecated 2 years ago.

Details and comments

#1720

[hhorii]

Even If we remove qobj supports in arguments, qobj will be still necessary to execute pulse simulation and DASK-based qasm simulation. That is, assemble() will be internally called in run(). To generate qobj for pulse, arguments for assemble() is necessary. For example, verify_wheel.py executes pulse simulation as follows

    qobj = assemble([schedule],
                    backend=backend_sim,
                    qubit_lo_freq=[5.0],
                    meas_level=1,
                    meas_return='avg',
                    shots=1)
    results = backend_sim.run(qobj, system_model=system_model).result()

Note that pulse simulation requires different arguments for assemble() and run(). If we remove qobj supports, the same arguments will be specified and unfortunately pulse simulation doesn't work correctly.

I think that qobj should be removed after unsupport of pulse simulation (#1722).