Revert D27478225: [pytorch][PR] Added pow() on CPU for float16 & bfloat16
Test Plan: revert-hammer
Differential Revision:
D27478225 (https://github.com/pytorch/pytorch/commit/6d030c14cf4bd2e59b6e4899a9ee4645d2199e68)
Original commit changeset: d309dd98d5a9
fbshipit-source-id: e0518f15185b41946caf3a8456c7af3f52e5a910
