MAGETWO-87974: Can't hide product images via hide_from_product_page attribute during import

- Removed show_on_product_page field form export/import;

Author: nikshostko

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

@@ -1241,7 +1241,6 @@ private function appendMultirowData(&$dataRow, $multiRawData)
             $dataRow = $this->rowCustomizer->addData($dataRow, $productId);
         } else {
             $additionalImageIsDisabled = [];
-            $additionalImageIsEnabled = [];
             if (!empty($multiRawData['mediaGalery'][$productLinkId])) {
                 foreach ($multiRawData['mediaGalery'][$productLinkId] as $mediaItem) {
                     if ((int)$mediaItem['_media_store_id'] === $storeId) {