GitLab

Summary:
This PR removes the legacy backend switch mechanism.
The implementation itself is still available.

Merge after v2.1 release

Pull Request resolved: https://github.com/pytorch/audio/pull/3559

Reviewed By: nateanl

Differential Revision: D48353764

Pulled By: mthrok

fbshipit-source-id: 4d3924dbe6f334ecebe2b12fcd4591c61c4aa656