[benchmark] Delete WORKSPACE bzl files (llvm#84013)

As like 48d8684, `WORKSPACE` is another
bazel-specific file that is unused. LLVM's bazel configuration is
entirely in `utils/bazel`.

Author: rupprecht

third-party/benchmark/WORKSPACE

@@ -13,4 +13,5 @@ git clone https://github.com/google/benchmark.git
 rm -rf benchmark/.git*
 find benchmark/ -name BUILD -delete
 find benchmark/ -name BUILD.bazel -delete
+find benchmark/ -name WORKSPACE -delete