drive-menu: Remove unused style class

The .single-indicator class was added to add extra spacing to
lone top bar items when removing the arrows, but dropped again
after solving the extra spacing differently.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/161>

extensions/drive-menu/extension.js

@@ -129,7 +129,7 @@ class DriveMenu extends PanelMenu.Button {
 
         let icon = new St.Icon({
             icon_name: 'media-eject-symbolic',
-            style_class: 'system-status-icon single-indicator',
+            style_class: 'system-status-icon',
         });
 
         this.add_child(icon);