status/system: Use icon-name property

The quick settings code was first developed in an extension, which
meant it made sense to maintain compatibility with GNOME 42 and not
use the new :icon-name convenience property.

There is no good for sticking with that for GNOME 43 as well.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2439>

js/ui/status/system.js

@@ -134,9 +134,7 @@ class ShutdownItem extends QuickSettingsItem {
             style_class: 'icon-button',
             hasMenu: true,
             canFocus: true,
-            child: new St.Icon({
             icon_name: 'system-shutdown-symbolic',
-            }),
             accessible_name: _('Power Off Menu'),
         });
 
@@ -210,9 +208,7 @@ class LockItem extends QuickSettingsItem {
         super._init({
             style_class: 'icon-button',
             can_focus: true,
-            child: new St.Icon({
             icon_name: 'system-lock-screen-symbolic',
-            }),
             accessible_name: _('Lock Screen'),
         });