remove buck2 from custom variables (#4231)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4231

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 variable doc.

Reviewed By: kirklandsign

Differential Revision: D59658382

fbshipit-source-id: 11026e30f03afd0fa4e29fccd48f7176c56ddad9