cleanup: Remove some unhelpful unused arguments

Those unused arguments aren't bugs - unbeknownst to eslint, they all
correspond to valid signal parameters - but they don't contribute
anything to clarity, so just remove them anyway.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/627

js/ui/extensionDownloader.js

@@ -203,12 +203,12 @@ class InstallExtensionDialog extends ModalDialog.ModalDialog {
         this.contentLayout.add(content);
     }
 
-    _onCancelButtonPressed(button, event) {
+    _onCancelButtonPressed() {
         this.close();
         this._invocation.return_value(GLib.Variant.new('(s)', ['cancelled']));
     }
 
-    _onInstallButtonPressed(button, event) {
+    _onInstallButtonPressed() {
         let params = { shell_version: Config.PACKAGE_VERSION };
 
         let url = REPOSITORY_URL_DOWNLOAD.format(this._uuid);