[Polly] Remove ability to use external shell

The external shell is getting removed soon, so remove the ability to
force enable it for polly tests.

Reviewers: ilovepi, Meinersbur, petrhosek

Pull Request: https://github.com/llvm/llvm-project/pull/209566

GitOrigin-RevId: 6e6272088e69834fde6e30f0033f36ee63806d00