Remove unused variable to fix clang compilation err (#4893)

Summary:
as title.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4893

Differential Revision: D13716733

Pulled By: riversand963

fbshipit-source-id: 6811d6a99fe2094d5344f854e8939f01238b2adb

utilities/transactions/write_prepared_transaction_test.cc

@@ -1199,7 +1199,6 @@ TEST_P(WritePreparedTransactionTest, MaxCatchupWithNewSnapshot) {
   const size_t commit_cache_bits = 0;    // only 1 entry => frequent eviction
   DestroyAndReopenWithExtraOptions(snapshot_cache_bits, commit_cache_bits);
   WriteOptions woptions;
-  TransactionOptions txn_options;
   WritePreparedTxnDB* wp_db = dynamic_cast<WritePreparedTxnDB*>(db);
 
   const int writes = 50;