Removed unused var

Author: Stanislav Idolov

app/code/Magento/CatalogImportExport/Model/Export/Product.php

@@ -768,7 +768,6 @@ protected function getItemsPerPage()
         if ($this->_itemsPerPage === null) {
             $memoryLimitConfigValue = trim(ini_get('memory_limit'));
             $lastMemoryLimitLetter = strtolower($memoryLimitConfigValue[strlen($memoryLimitConfigValue) - 1]);
-            $memoryUsage = memory_get_usage(true);
             $memoryLimit = (int) $memoryLimitConfigValue;
             switch ($lastMemoryLimitLetter) {
                 case 'g':