panel-list: Further tweak sidebar classes

Removes sidebar-label class, adds sidebar-icon class to
Devices and Details arrows

https://bugzilla.gnome.org/show_bug.cgi?id=786612

shell/cc-panel-list.c

@@ -194,7 +194,6 @@ row_data_new (CcPanelCategory  category,
                         "xalign", 0.0,
                         "hexpand", TRUE,
                         NULL);
-  gtk_style_context_add_class (gtk_widget_get_style_context (label), "sidebar-label");
   gtk_grid_attach (GTK_GRID (grid), label, 1, 0, 1, 1);
 
   /* Description label */