Cleanup selective build example options from root CMakeLists.txt (#1381)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1381

`EXECUTORCH_SELECT_OPS_LIST` is being used by arm example and
selective build example. I'm adding that option to both examples'
CMakeLists.txt and remove it from the root level one.

Reviewed By: lucylq

Differential Revision: D52051297

fbshipit-source-id: ae49a44366d6f73792ab626d4bd19b085fe8c60b