Stop polling the background symlink every 100ms

The background plugin ran a 100ms Timer that spawned a readlink
subprocess ~10x/sec for the life of the shell, purely to notice
wallpaper changes. Wallpaper updates already arrive over IPC
(omarchy-theme-bg-set calls `background set`, omarchy-theme-set calls
`background themeTransition`), and the initial wallpaper is read once
in Component.onCompleted, so the poll was redundant.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

shell/plugins/background/Background.qml

@@ -151,13 +151,6 @@ Item {
     }
   }
 
-  Timer {
-    interval: 100
-    running: true
-    repeat: true
-    onTriggered: root.refreshBackground()
-  }
-
   Timer {
     id: pendingThemeFallbackTimer
     interval: 300