Revert "Reland "[benchmarks] Move libcxx's fork of google/benchmark and llvm/utils'"""

This reverts commit 1ee32055.

We hit additional bot failures; in particular, Fuchsia's seems to be
related to how CMakeLists are ingested, see https://ci.chromium.org/ui/p/fuchsia/builders/toolchain.ci/clang-linux-x64/b8830380874445931681/overview