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/ibusCandidatePopup.js

@@ -228,7 +228,7 @@ class IbusCandidatePopup extends BoxPointer.BoxPointer {
                     rect.width,
                     rect.height);
             });
-        } catch (e) {
+        } catch {
             // Only recent IBus versions have support for this signal
             // which is used for wayland clients. In order to work
             // with older IBus versions we can silently ignore the