Remove stale migration guide from docs

[ElePT]

Custom runtime programs are long gone, making the migration guide stale: https://qiskit.github.io/qiskit-serverless/migration/migration_from_qiskit_runtime_programs.html. I suggest removing it in an attempt to keep the docs more up to date.

[calebjubal]

Can I work on this issue?

[ElePT]

Sure, I'll assign you.

[calebjubal]

Hey @ElePT and @avilches should I go with the suggestion or is there any other alternative in process?

[ElePT]

I don't think there is much more beyond what was suggested in the issue. The only open question is whether to keep the migration guides section at all (even if empty), or remove it and potentially re-add it if we need it in the future (I am leaning towards that one).

[Tansito]

Yes, let's remove it 👍