Remove ifdef for try_emplace after upgrading to c++17 (#9932)

Summary:
Test plan
make check

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9932

Reviewed By: ajkr

Differential Revision: D36085404

Pulled By: riversand963

fbshipit-source-id: 2ece14ca0e2e4c1288339ff79e7e126b76eaf786

utilities/transactions/lock/point/point_lock_tracker.cc

@@ -44,25 +44,12 @@ class TrackedKeysIterator : public LockTracker::KeyIterator {
 
 void PointLockTracker::Track(const PointLockRequest& r) {
   auto& keys = tracked_keys_[r.column_family_id];
-#ifdef __cpp_lib_unordered_map_try_emplace
-  // use c++17's try_emplace if available, to avoid rehashing the key
-  // in case it is not already in the map
   auto result = keys.try_emplace(r.key, r.seq);
   auto it = result.first;
   if (!result.second && r.seq < it->second.seq) {
     // Now tracking this key with an earlier sequence number
     it->second.seq = r.seq;
   }
-#else
-  auto it = keys.find(r.key);
-  if (it == keys.end()) {
-    auto result = keys.emplace(r.key, TrackedKeyInfo(r.seq));
-    it = result.first;
-  } else if (r.seq < it->second.seq) {
-    // Now tracking this key with an earlier sequence number
-    it->second.seq = r.seq;
-  }
-#endif
   // else we do not update the seq. The smaller the tracked seq, the stronger it
   // the guarantee since it implies from the seq onward there has not been a
   // concurrent update to the key. So we update the seq if it implies stronger