Revert unneeded fixes in flat_hash_map (pytorch#16907)

Summary:
Pull Request resolved: pytorch#16907

The begin()/end() fix actually doesn't make sense, see my comment on skarupke/flat_hash_map#8
This diff removes it.

Reviewed By: ezyang

Differential Revision: D13985779

fbshipit-source-id: f08b02c941069e2a4e728e02a19b65dc72f96b41

c10/util/flat_hash_map.h

@@ -1,6 +1,5 @@
 // Taken from https://github.com/skarupke/flat_hash_map/blob/2c4687431f978f02a3780e24b8b701d22aa32d9c/flat_hash_map.hpp
 // with fixes applied:
-// - https://github.com/skarupke/flat_hash_map/pull/8
 // - https://github.com/skarupke/flat_hash_map/pull/25
 // - https://github.com/skarupke/flat_hash_map/pull/26
 // - replace size_t with uint64_t to fix it for 32bit
@@ -522,7 +521,6 @@ class sherwood_v3_table : private EntryAlloc, private Hasher, private Equal
             if (it->has_value())
                 return { it };
         }
-        return end();
     }
     const_iterator begin() const
     {
@@ -531,7 +529,6 @@ class sherwood_v3_table : private EntryAlloc, private Hasher, private Equal
             if (it->has_value())
                 return { it };
         }
-        return end();
     }
     const_iterator cbegin() const
     {