[benchmark] Delete BUILD files

Bazel support is through utils/bazel, and the BUILD files in `benchmark`
just complicate downstream integrates for bazel based builds.

Differential Revision: https://reviews.llvm.org/D115733

Author: mtrofin

third-party/benchmark/BUILD.bazel

@@ -11,4 +11,6 @@ read -p "Press a key to continue, or Ctrl+C to cancel"
 rm -rf benchmark
 git clone https://github.com/google/benchmark.git
 rm -rf benchmark/.git*
+find benchmark/ -name BUILD -delete
+find benchmark/ -name BUILD.bazel -delete