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/aos/flatbuffers.h b/aos/flatbuffers.h
index b48dc1f..ff2dced 100644
--- a/aos/flatbuffers.h
+++ b/aos/flatbuffers.h
@@ -144,11 +144,7 @@
   const uint8_t *data() const override {
     return reinterpret_cast<const uint8_t *>(data_.data());
   }
-  uint8_t *data() override {
-    // TODO(james): when we get c++17, can we drop the second cast?
-    return const_cast<uint8_t *>(
-        reinterpret_cast<const uint8_t *>(data_.data()));
-  }
+  uint8_t *data() override { return reinterpret_cast<uint8_t *>(data_.data()); }
   size_t size() const override { return data_.size(); }
 
  private: