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/sessionMode.js

@@ -112,7 +112,7 @@ function _loadMode(file, info) {
         [success_, fileContent] = file.load_contents(null);
         const decoder = new TextDecoder();
         newMode = JSON.parse(decoder.decode(fileContent));
-    } catch (e) {
+    } catch {
         return;
     }