Revert D29998114: [pytorch][PR] enable bf16 mkldnn path for gemm
Test Plan: revert-hammer
Differential Revision:
D29998114 (https://github.com/pytorch/pytorch/commit/acc9f9afc8f2be70d7f5d3248ca1760e0336b3b8)
Original commit changeset: 459dc5874c63
fbshipit-source-id: 1994623a3afc22a94bd0cf5de766b023185f5238
