Remove legacy tosa backed - which is now landed as arm

Summary:
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: 8878d57afe9afb349c5d1e4053f4f3b19bd63d1c

Author: digantdesai