Migrate `qiskit-ibmq-provider`

[Guillermo-Mijares-Vilarino]

Summary

This PR continues the migration of the qiskit-ibmq-provider repository documentation to a separate page (Qiskit/qiskit-ibmq-provider#1166), this time from the metapackage side.

Details and comments

• Remove qiskit-ibmq-provider from the list of API references built by the metapackage.
• Remove the deprecate_ibmq_provider custom extension (it will no longer be necessary)
• Update the link to the documentation (the new page will be created by Qiskit/qiskit-ibmq-provider#1166)
• Remove the section about installing qiskit-ibmq-provider from the getting started.
• Remove note about the RuntimeWarning from terra when aer and ibmq-provider are not installed. See Qiskit/qiskit-terra#5619.
• Didn't touch the reference to the configuration variables QISKIT_IBMQ_PROVIDER_LOG_LEVEL and QISKIT_IBMQ_PROVIDER_LOG_FILE from docs/configuration.rst in case these still exist.

Blocked by Qiskit/qiskit-ibmq-provider#1166

[Guillermo-Mijares-Vilarino]

I believe this is ready to review but still blocked by https://github.com/Qiskit/qiskit-ibmq-provider/pull/1166

[HuangJunye]

On hold until https://github.com/Qiskit/qiskit-ibmq-provider/pull/1166 is merged and ibmq provider's own documentation page goes live.

[Guillermo-Mijares-Vilarino]

Now that Qiskit/qiskit-ibmq-provider#1166 is merged we can remove the on hold label.