keyboard: Ensure completion is disabled when OSK is destroyed

If the OSK is destroyed while visible without being close()'d first,
the completion mode might remain turned on. Ensure it is turned off
on OSK destruction so that typing-booster has no chance to remain
turned on.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2543>

js/ui/keyboard.js

@@ -1388,6 +1388,8 @@ var Keyboard = GObject.registerClass({
             this._languagePopup.destroy();
             this._languagePopup = null;
         }
+
+        IBusManager.getIBusManager().setCompletionEnabled(false, () => Main.inputMethod.update());
     }
 
     _setupKeyboard() {