Remove references to prototype tutorials (#3171)

Author: hwangjeff

docs/source/index.rst

@@ -51,11 +51,6 @@ model implementations and application components.
    tutorials/audio_feature_extractions_tutorial
    tutorials/audio_feature_augmentation_tutorial
 
-   tutorials/oscillator_tutorial
-   tutorials/additive_synthesis_tutorial
-   tutorials/filter_design_tutorial
-   tutorials/subtractive_synthesis_tutorial
-
    tutorials/audio_datasets_tutorial
 
 .. toctree::
@@ -189,34 +184,6 @@ Tutorials
    :link: tutorials/audio_feature_augmentation_tutorial.html
    :tags: Preprocessing
 
-.. customcarditem::
-   :header: Generating waveforms with oscillator
-   :card_description:
-   :image: _images/sphx_glr_oscillator_tutorial_003.png
-   :link: tutorials/oscillator_tutorial.html
-   :tags: DSP
-
-.. customcarditem::
-   :header: Additive Synthesis
-   :card_description:
-   :image: _images/sphx_glr_additive_synthesis_tutorial_001.png
-   :link: tutorials/additive_synthesis_tutorial.html
-   :tags: DSP
-
-.. customcarditem::
-   :header: Designing digital filters
-   :card_description:
-   :image: _images/sphx_glr_filter_design_tutorial_001.png
-   :link: tutorials/filter_design_tutorial.html
-   :tags: DSP
-
-.. customcarditem::
-   :header: Subtractive Synthesis
-   :card_description:
-   :image: _images/sphx_glr_subtractive_synthesis_tutorial_002.png
-   :link: tutorials/subtractive_synthesis_tutorial.html
-   :tags: DSP
-
 .. customcarditem::
    :header: Audio dataset
    :card_description: Learn how to use <code>torchaudio.datasets</code> module.