Remove legacy `tosa` backed - which is now landed as `arm` (#464)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/464

This was introduced in PR#161 but for some reason cause a merge conflict.
Landing it as a stand alone diff.

Reviewed By: mergennachin

Differential Revision: D49547823

fbshipit-source-id: 1fb05fcaaaa140640a1bfdff621e115967daa768