Fix linting error

lib/api/encodeProgress.js

@@ -9,11 +9,6 @@ var encodeNavigation = require('./encodeNavigation');
     @return {Object}
 */
 function encodeProgress(output, page) {
-    var book = output.getBook();
-    var pages = output.getPages();
-    var summary = book.getSummary();
-    var articles = summary.getArticlesAsList();
-
     var current = page.getPath();
     var navigation = encodeNavigation(output);
     navigation = Immutable.Map(navigation);