chore(parler-tts): drop backend (#4672)

We support at this point more extensive backends that are SOTA and
support also voice cloning, and many other features. This backend is
superseded and also poses significant maintenance burden as there is an
open issue https://github.com/mudler/LocalAI/issues/3941 which is still
open as it deps are pinning old versions of grpc.

Closes https://github.com/mudler/LocalAI/issues/3941

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>