CoreML delegate scripts cleanup (#1025)

Summary:
- Removed `install_requirements_internal script`. The script was cloning a private coremltools repo, executorch changes are now in the public coremltools repo.
- Xcode projects were referring to the parent directory of executorch, the `include` path becomes invalid if the directory name is not executorch. Updated the scripts to copy the required executorch headers.
- Updated coreml-delegate-test to use the build_all script. The script builds and runs the tests and the executor.

Pull Request resolved: https://github.com/pytorch/executorch/pull/1025

Reviewed By: cccclai

Differential Revision: D50479135

Pulled By: shoumikhin

fbshipit-source-id: f9908e58bcd9ae8882c051afb8adabca61642fef