[Bazel] Don't fail the build on usage of deprecated APIs

Build failures are not a particularly helpful way to enforce not using
deprecated APIs and that isn't the point of the Bazel build.

At the same time, this removes `-Wno-unused` this is a check that we do
enforce in the Google internal build and so are ok maintaining in our
maintenance of the upstream Bazel build (the comment about not wanting
to do so was from a time when this was in a separate repository and I was
the only one maintaining it).

Differential Revision: https://reviews.llvm.org/D118671