cut std::hash specialization for int128

Summary:
Cut `std::hash` specialization in folly for `signed __int128` and `unsigned __int128`.

There are cases where folly does polyfill things in the global and standard namespaces, but in order for folly to play nicely with other libraries this practice is overall to be avoided.

Reviewed By: nlutsenko

Differential Revision: D26455137

fbshipit-source-id: 948140513b8daf6fe777d04ea42346d69c5eb03f

folly/hash/Hash.h

@@ -751,15 +751,6 @@ struct TupleHasher<0, Ts...> {
 
 // Custom hash functions.
 namespace std {
-#if FOLLY_SUPPLY_MISSING_INT128_TRAITS
-template <>
-struct hash<__int128> : folly::detail::integral_hasher<__int128> {};
-
-template <>
-struct hash<unsigned __int128>
-    : folly::detail::integral_hasher<unsigned __int128> {};
-#endif
-
 // Hash function for pairs. Requires default hash functions for both
 // items in the pair.
 template <typename T1, typename T2>