Revert D26906509: Adding parallel support for the LLVM backend.
Test Plan: revert-hammer
Differential Revision:
D26906509 (https://github.com/pytorch/pytorch/commit/95d2318510371523b3406ae1d4818f8f0607bbc6)
Original commit changeset: 12c17f2f21af
fbshipit-source-id: cc86d0dfca0dd791b31bda23a0172fc1cfd89760
