Remove circuit library provider from the sidebar

[1ucian0]

Currently, the side bar includes Circuit Library (in a singleton section called Libraries) as part of the sidebar.

The original goal was to highlight this feature. However, they link to the feature now is in the landing page, in much more prominent place:

This fulfils the original goal and the singleton section can be removed.

Does existing documentation need to be updated with new content?

• Which files need to be updated?

docs/index.rst

• What content needs to be updated or replaced? (You can copy and paste)

  API Reference <apidoc/terra>
- circuit_library
  aer
  ibmq-provider
  release_notes
  configuration

• What new content needs to be added?

None.

[mtreinish]

To do this you will need to update terra, the circuit library is managed as a separate table of contents and source file in the apidocs pulled in from terra.

[HuangJunye]

This can be done after the next Terra release. See more details https://github.com/Qiskit/qiskit/pull/1638#issuecomment-1421070966