Remove unused extension/aot_util directory (#3216)

Summary:
The AOT util extension was removed a while back, but the directory and README still exist. This PR cleans them up. Note that the aot_util sources were deleted previously, so this is not a functional change.

Pull Request resolved: https://github.com/pytorch/executorch/pull/3216

Test Plan: CI. This is not a functional change, as it changes only a README file.

Reviewed By: metascroy

Differential Revision: D56436216

Pulled By: GregoryComer

fbshipit-source-id: 2f8b8cee20b7a3efb25a1ef1df3ebd69f3b512c9