Remove dead code, uninitialized and unused variables in cache_resize cpp test

CID 122319 (#1 of 1): Uninitialized scalar variable (UNINIT)

uninit_use: Using uninitialized value prev_txn_id.

213            } else if (prev_txn_id != tracked_txn_id) {

   CID 122323 (#1 of 1): Logically dead code (DEADCODE)

dead_error_line: Execution cannot reach this statement:

212                first_record = false;

CID 122317 (#1 of 1): Unused value (UNUSED_VALUE)

returned_value: Assigning value from std::stoull(std::string const(tracked_cache_size, std::allocator<char>()), NULL, 10) to cache_size here, but that stored value is not used.

226            cache_size = std::stoull(tracked_cache_size);