Remove stale TODO

Summary: This was fixed by 0c2bd5cb

Test Plan: n/a

Reviewers: gabijs

Reviewed By: gabijs

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D54753

include/rocksdb/utilities/transaction_db.h

@@ -92,8 +92,6 @@ struct TransactionOptions {
   // will never relinquish any locks it holds.  This could prevent keys from
   // being
   // written by other writers.
-  //
-  // TODO(agiardullo):  Improve performance of checking expiration time.
   int64_t expiration = -1;
 };