Do not make visible close tab button on small screen

The navbar where there is already a close button is collapsed only on extra
small screen.

issue8291
review267271002

src/tab.js

@@ -180,7 +180,7 @@
                 'role': 'menu'
             })).append(jQuery('<button/>', {
                 'type': 'button',
-                'class': 'close visible-xs visible-sm',
+                'class': 'close visible-xs',
                 'aria-label': Sao.i18n.gettext('Close')
             }).append(jQuery('<span/>', {
                 'aria-hidden': true