Revert "background: Mark pattern backgrounds as loaded"

This reverts commit fdac0602db.

The commit was fixing a crash, which cannot longer be reproduced, but also
introduced a noticeable white flash when switching wallpapers. This will
become even more noticeable when we try to uniformly transition the whole
screen.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2070>

js/ui/background.js

@@ -365,8 +365,6 @@ var Background = GObject.registerClass({
             this.set_color(color);
         else
             this.set_gradient(shadingType, color, secondColor);
-
-        this._setLoaded();
     }
 
     _watchFile(file) {