Remove the redundant install_requirement script when test iOS on CI (#1583)

Summary:
All the dependency has already been setup by https://github.com/pytorch/executorch/blob/main/.github/workflows/app-build.yml#L40.  Running the install script again here will overwrite the PyTorch version causing version mismatched failure, i.e. https://github.com/pytorch/executorch/actions/runs/7483610205/job/20369124052

Pull Request resolved: https://github.com/pytorch/executorch/pull/1583

Reviewed By: shoumikhin

Differential Revision: D52684492

Pulled By: huydhn

fbshipit-source-id: b90a29d96a84e7e3b5c6f5b29b468786d5bcc332