Remove unused backend code

[jyu00]

Summary

Some more cleanup

• Remove apiconstants.py, which is only used for the legacy job path (with the exception of QISKIT_IBM_RUNTIME_API_URL, which is moved to constants.py)
• Remove backendjoblimit.py since runtime doesn't have job limits
• Raise an error in backend.run()
• Remove _deprecate_id_instruction() because it's only used in backend.run().

We may need to remove backend reservation as well since it's not supported by cloud runtime and isn't really related to runtime.

Details and comments

Fixes #

[rathishcholarajan]

Looks good! Thanks @jyu00!

[rathishcholarajan]

@jyu00 Looks good. Could you please fix lint?

[daka1510]

FYI: I've hit the same linting error in my other PR and fixed it already via ac4f27c.

************* Module qiskit_ibm_runtime.runtime_program
qiskit_ibm_runtime/runtime_program.py:17:0: W0611: Unused List imported from typing (unused-import)

Neither Jessie, nor I have touched qiskit_ibm_runtime/runtime_program so I'm not sure why the unused import wasn't caught earlier.

@rathishcholarajan given the above and that we're in an early stage of this repo: do you think we can go ahead and integrate this refactoring and trust that the linter issue will be fixed with my PR in a next step :)?

[rathishcholarajan]

FYI: I've hit the same linting error in my other PR and fixed it already via ac4f27c.

************* Module qiskit_ibm_runtime.runtime_program
qiskit_ibm_runtime/runtime_program.py:17:0: W0611: Unused List imported from typing (unused-import)

Neither Jessie, nor I have touched qiskit_ibm_runtime/runtime_program so I'm not sure why the unused import wasn't caught earlier.

@rathishcholarajan given the above and that we're in an early stage of this repo: do you think we can go ahead and integrate this refactoring and trust that the linter issue will be fixed with my PR in a next step :)?

Thanks @daka1510! That is weird. We need to investigate if this happens again.