Check for manual_sorting feature flag

app/views/shared/issuable/_sort_dropdown.html.haml

@@ -17,6 +17,6 @@
           = sortable_item(sort_title_due_date,          page_filter_path(sort: sort_value_due_date),          sort_title) if viewing_issues
           = sortable_item(sort_title_popularity,        page_filter_path(sort: sort_value_popularity),        sort_title)
           = sortable_item(sort_title_label_priority,    page_filter_path(sort: sort_value_label_priority),    sort_title)
-          = sortable_item(sort_title_relative_position, page_filter_path(sort: sort_value_relative_position), sort_title) if viewing_issues
+          = sortable_item(sort_title_relative_position, page_filter_path(sort: sort_value_relative_position), sort_title) if viewing_issues && Feature.enabled?(:manual_sorting)
           = render_if_exists('shared/ee/issuable/sort_dropdown', viewing_issues: viewing_issues, sort_title: sort_title)
     = issuable_sort_direction_button(sort_value)