fix: remove unused variable (XRPLF#4677)

With clang 15, an unused-but-set-variable warning was emitted:

PostgresDatabase.cpp:178:14: warning: variable 'expNumResults' set but
not used [-Wunused-but-set-variable]
    uint32_t expNumResults = 1;

src/ripple/app/rdb/backend/impl/PostgresDatabase.cpp

@@ -175,8 +175,6 @@ loadLedgerInfos(
            "total_coins, closing_time, prev_closing_time, close_time_res, "
            "close_flags, ledger_seq FROM ledgers ";
 
-    uint32_t expNumResults = 1;
-
     if (auto ledgerSeq = std::get_if<uint32_t>(&whichLedger))
     {
         sql << "WHERE ledger_seq = " + std::to_string(*ledgerSeq);
@@ -189,8 +187,6 @@ loadLedgerInfos(
         auto minAndMax =
             std::get_if<std::pair<uint32_t, uint32_t>>(&whichLedger))
     {
-        expNumResults = minAndMax->second - minAndMax->first;
-
         sql
             << ("WHERE ledger_seq >= " + std::to_string(minAndMax->first) +
                 " AND ledger_seq <= " + std::to_string(minAndMax->second));