remove buck2 from mps readme (#4232)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4232

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  mps backend readme

Reviewed By: kirklandsign

Differential Revision: D59659285

fbshipit-source-id: 3e58e49e9c7838193d6b91ae550af2fbb0174dda