js: Remove unused catch bindings

Optional catch bindings have been supported for quite a while now,
so we can treat unused error bindings in catch statements like any
other unused variable.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3684>

js/ui/background.js

@@ -503,7 +503,7 @@ const Background = GObject.registerClass({
                 Gio.FileQueryInfoFlags.NONE,
                 0,
                 this._cancellable);
-        } catch (e) {
+        } catch {
             this._setLoaded();
             return;
         }