Remove unused files.

xpfe/components/sidebar/resources/MANIFEST-skin

@@ -1,6 +1,5 @@
 customize-panel.css
 customize.css
-flash.css
 sidebarOverlay.css
 list-down-dis.gif
 list-down.gif
@@ -8,6 +7,5 @@ list-up-dis.gif
 list-up.gif
 panel-collapse.gif
 panel-expand.gif
-alert.gif
 sidebar-bottomright-cap.gif
 sidebar-topright-cap.gif