Remove undefined _translateModel property

The lines were a reminiscence from when the translation logic was in the
core module. The logic was replaced with the correct use of the
inlineTranslation property that is also present at the end of the method.

app/code/Magento/Sitemap/Model/Observer.php

@@ -121,8 +121,7 @@ public function scheduledGenerateSitemaps()
             \Magento\Store\Model\ScopeInterface::SCOPE_STORE
         )
         ) {
-            $translate = $this->_translateModel->getTranslateInline();
-            $this->_translateModel->setTranslateInline(false);
+            $this->inlineTranslation->suspend();
 
             $this->_transportBuilder->setTemplateIdentifier(
                 $this->_scopeConfig->getValue(