[BC-Breaking] Remove legacy global backend switch

[mthrok]

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

Merge after v2.1 release

[pytorch-bot]

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This pull request was exported from Phabricator. Differential Revision: D48353764

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This pull request was exported from Phabricator. Differential Revision: D48353764

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This pull request was exported from Phabricator. Differential Revision: D48353764

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This pull request was exported from Phabricator. Differential Revision: D48353764

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This pull request was exported from Phabricator. Differential Revision: D48353764

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This pull request was exported from Phabricator. Differential Revision: D48353764

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@mthrok merged this pull request in 454418d.

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Hey @mthrok.
You merged this PR, but labels were not properly added. Please add a primary and secondary label (See https://github.com/pytorch/audio/blob/main/.github/process_commit.py).

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