Remove outdated doc (#2617)

Summary:
`ctc_decoder` has become beta, remove it from prototype documents.

Pull Request resolved: https://github.com/pytorch/audio/pull/2617

Reviewed By: hwangjeff

Differential Revision: D38706869

Pulled By: nateanl

fbshipit-source-id: 41679f4e65a584b6b882af4551a50123f1dcef02

docs/source/index.rst

@@ -60,7 +60,6 @@ Prototype API References
    :caption: Prototype API Reference
 
    prototype
-   prototype.ctc_decoder
    prototype.functional
    prototype.models
    prototype.pipelines