Remove buck2 from selective build readme (#4003)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4003

However, keep the description / documentation inside test_selective_build.sh for internal users.

Reviewed By: Gasoonjia

Differential Revision: D58706163

fbshipit-source-id: 5aae1419d87c5cdf08f91c0a1e73ef5788b018cd