Remove passing const variable to thread (#5443)

Summary:
CLANG complains that passing const to thread is not necessary. The patch removes it form PreparedHeap::Concurrent test.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5443

Differential Revision: D15781598

Pulled By: maysamyabandeh

fbshipit-source-id: 3aceb05d96182fa4726d6d37eed45fd3aac4c016

utilities/transactions/write_prepared_transaction_test.cc

@@ -164,7 +164,7 @@ TEST(PreparedHeap, Concurrent) {
 
   for (size_t n = 0; n < 100; n++) {
     last = 0;
-    t[0] = rocksdb::port::Thread([&heap, t_cnt, &last]() {
+    t[0] = rocksdb::port::Thread([&heap, &last]() {
       Random rnd(1103);
       for (size_t seq = 1; seq <= t_cnt; seq++) {
         // This is not recommended usage but we should be resilient against it.