Remove hdemucs init from prototype (#2817)

Summary:
Now that hybrid demucs is officially released as beta, remove it's temp prototype initialization support

Pull Request resolved: https://github.com/pytorch/audio/pull/2817

Reviewed By: mthrok

Differential Revision: D40908696

Pulled By: carolineechen

fbshipit-source-id: bc87a4b7aeb27db00e10bdce91cd71688cb08769

torchaudio/prototype/models/hdemucs/__init__.py

-import torchaudio
-
-functions = ["HDemucs", "hdemucs_high", "hdemucs_medium", "hdemucs_low"]
-
-
-def __getattr__(name: str):
-    if name in functions:
-        import warnings
-
-        warnings.warn(
-            f"{__name__}.{name} has been moved to torchaudio.models.hdemucs",
-            DeprecationWarning,
-        )
-
-        return getattr(torchaudio.models, name)
-
-    raise AttributeError(f"module {__name__} has no attribute {name}")
-
-
-def __dir__():
-    return functions