Revert D8304491: [folly] Use folly Hash to hash strings in dynamic

Differential Revision:
D8304491

Original commit changeset: 9988e9012c31

fbshipit-source-id: cc88b31cfb65e3dbc0a2b8f7fbcc565a28a796f9

folly/dynamic.cpp

@@ -315,8 +315,11 @@ std::size_t dynamic::hash() const {
     return std::hash<double>()(getDouble());
   case BOOL:
     return std::hash<bool>()(getBool());
-  case STRING:
-    return Hash()(getString());
+  case STRING: {
+    // keep it compatible with FBString
+    const auto& str = getString();
+    return ::folly::hash::fnv32_buf(str.data(), str.size());
+  }
   }
   assume_unreachable();
 }