remove duplicate generate_lib_aten target under aten kernel (#2951)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2951

generate_lib and generate_lib_aten are exactly the same under executorch/kernels/aten. Remove the generate_lib_aten for better understanding.

Reviewed By: larryliu0820

Differential Revision: D55937122

fbshipit-source-id: 5e7e7c06efbd4876874880627b67934d782473a2