Revert "turn on -Werror=unused-function in our Bazel CPU build"
This reverts commit 67d313a03259be4da7a1d623a9df6791e02248e8.
Reverted https://github.com/pytorch/pytorch/pull/79154 on behalf of https://github.com/malfet due to Breaks bazel build: https://hud.pytorch.org/pytorch/pytorch/commit/67d313a03259be4da7a1d623a9df6791e02248e8
