Revert "std: Cache HashMap keys in TLS"

This reverts commit eaeef3d.

This is a short-term workaround to rust-lang#36481.

src/libstd/collections/hash/map.rs

@@ -1963,33 +1963,8 @@ impl RandomState {
     #[allow(deprecated)] // rand
     #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
     pub fn new() -> RandomState {
-        // Historically this function did not cache keys from the OS and instead
-        // simply always called `rand::thread_rng().gen()` twice. In #31356 it
-        // was discovered, however, that because we re-seed the thread-local RNG
-        // from the OS periodically that this can cause excessive slowdown when
-        // many hash maps are created on a thread. To solve this performance
-        // trap we cache the first set of randomly generated keys per-thread.
-        //
-        // In doing this, however, we lose the property that all hash maps have
-        // nondeterministic iteration order as all of those created on the same
-        // thread would have the same hash keys. This property has been nice in
-        // the past as it allows for maximal flexibility in the implementation
-        // of `HashMap` itself.
-        //
-        // The constraint here (if there even is one) is just that maps created
-        // on the same thread have the same iteration order, and that *may* be
-        // relied upon even though it is not a documented guarantee at all of
-        // the `HashMap` type. In any case we've decided that this is reasonable
-        // for now, so caching keys thread-locally seems fine.
-        thread_local!(static KEYS: (u64, u64) = {
-            let r = rand::OsRng::new();
-            let mut r = r.expect("failed to create an OS RNG");
-            (r.gen(), r.gen())
-        });
-
-        KEYS.with(|&(k0, k1)| {
-            RandomState { k0: k0, k1: k1 }
-        })
+        let mut r = rand::thread_rng();
+        RandomState { k0: r.gen(), k1: r.gen() }
     }
 }