Fix clang 15 -Wunused-but-set-variable warnings in MyRocks (percona#1304)

Upstream commit ID: facebook/mysql-5.6@4475c80
PS-8951: Merge percona-202305 (https://jira.percona.com/browse/PS-8951)

Summary:
- Make infinite debug loop counter volatile. Otherwise it's undefined behavior and/or the loop might be optimized away.
- Remove another write-only variable

Pull Request resolved: facebook/mysql-5.6#1304

Reviewed By: hermanlee

Differential Revision: D45446127

Pulled By: sunshine-Chun

fbshipit-source-id: c0dfddfa76b4004ae61d62b5de305cb3264afb1c

Author: laurynas-biveinis

storage/rocksdb/ha_rocksdb.cc

@@ -13167,7 +13167,6 @@ static int read_stats_from_ssts(
     }
   }
 
-  int num_sst [[maybe_unused]] = 0;
   for (const auto &it : props) {
     std::vector<Rdb_index_stats> sst_stats;
     Rdb_tbl_prop_coll::read_stats_from_tbl_props(it.second, &sst_stats);
@@ -13197,7 +13196,6 @@ static int read_stats_from_ssts(
       (*stats)[it1.m_gl_index_id].merge(
           it1, true, it_index->second->max_storage_fmt_length());
     }
-    num_sst++;
   }
 
   DBUG_RETURN(HA_EXIT_SUCCESS);