Clean up post-C++-17 upgrade

-There was a TODO to remove a const_cast
-Remove a bunch of -pthread uses, because they were apparently unneeded.

Change-Id: I4cd28ccfb487eb0c7acb3b629e090c5e8a1b2acc
diff --git a/third_party/google-benchmark/BUILD.bazel b/third_party/google-benchmark/BUILD.bazel
index 7ba1887..479d59e 100644
--- a/third_party/google-benchmark/BUILD.bazel
+++ b/third_party/google-benchmark/BUILD.bazel
@@ -24,7 +24,7 @@
     ],
     linkopts = select({
         ":windows": ["-DEFAULTLIB:shlwapi.lib"],
-        "//conditions:default": ["-pthread"],
+        "//conditions:default": [],
     }),
     strip_include_prefix = "include",
     visibility = ["//visibility:public"],