Back out "use ovrsource libtorch in executorch"

[h-friederich]

Summary: This is causing linker errors internally.

Differential Revision: D66570005

[pytorch-bot]

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❌ 1 New Failure

As of commit 6dc6307 with merge base 651af12 ():

NEW FAILURE - The following job has failed:

• Check Labels / Check labels (gh)
  RuntimeError: Error checking labels: PR does not have required labels

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This pull request was exported from Phabricator. Differential Revision: D66570005

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