Remove err when checking for note in save file

Note is not required to be present in any save file. However,
get_by_xpath() sets err if it is not, which will never be checked for.

src/ag-chart.c

@@ -771,7 +771,7 @@ ag_chart_load_from_file(GFile *file, GError **err)
             "/chartinfo/note/text()",
             FALSE,
             XML_CONVERT_STRING,
-            err
+            NULL
         );
 
     if (found_error) {