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>

tests/unit/markup.js

@@ -19,7 +19,7 @@ describe('fixMarkup()', () => {
                 function isMarkupValid(markup) {
                     try {
                         Pango.parse_markup(markup, -1, '');
-                    } catch (e) {
+                    } catch {
                         return false;
                     }
                     return true;