Revert D25399466: add channels last support for AvgPool2d on CPU
Test Plan: revert-hammer
Differential Revision:
D25399466 (https://github.com/pytorch/pytorch/commit/8ac0917cc7cb3666f50a3c4599e480470d916dda)
Original commit changeset: 9477b0c281c0
fbshipit-source-id: e0245f0e390f5eca228445fd82d6e5142a827abc
