remove old buck2 setup code (#3870)

Summary:
now fetching the tool is part of the buildsystem proper, we can just expunge references to buck2. Only bit I couldn't fully test is the github workflow which will happen in CI now.

Pull Request resolved: https://github.com/pytorch/executorch/pull/3870

Reviewed By: kirklandsign

Differential Revision: D58368002

Pulled By: digantdesai

fbshipit-source-id: 7e37e70f68c688ec0ae55e1b95443d1fba7ee9c5