unlockDialog: Don't destroy on cancel

Otherwise there will be no way to recover it in the future. Also
remove an else condition that assumed the dialog would destroy
itself on cancel.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/872

js/ui/screenShield.js

@@ -191,8 +191,6 @@ var ScreenShield = class {
             // on its own, so ensure it stays on lock screen
             // instead
             this._lockScreenGroup.grab_key_focus();
-        } else {
-            this._dialog = null;
         }
     }
 

js/ui/unlockDialog.js

@@ -499,8 +499,6 @@ var UnlockDialog = GObject.registerClass({
 
     cancel() {
         this._authPrompt.cancel();
-
-        this.destroy();
     }
 
     addCharacter(unichar) {