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/misc/systemActions.js

@@ -340,7 +340,7 @@ const SystemActions = GObject.registerClass({
         try {
             const [canShutdown] = await this._session.CanShutdownAsync();
             this._canHavePowerOff = canShutdown;
-        } catch (e) {
+        } catch {
             this._canHavePowerOff = false;
         }
         this._updatePowerOff();