remove buck2 from custom operator registration (#4233)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4233

For alpha+, we need to remove all buck2 commands and buck2 dependencies from static doc and github readmes.
This diff gets rid of the buck2 from  custom operator registration examples.

Reviewed By: larryliu0820

Differential Revision: D59661023

fbshipit-source-id: 25b4fec3f5ff71e615bc52b4b465d31a8e4c6c87