keyboard: Remove unused code

This signal does not exist, the Suggestions.add() method allows to attach
per-element callbacks instead.

js/ui/keyboard.js

@@ -675,9 +675,6 @@ var Keyboard = class Keyboard {
         this._current_page = null;
 
         this._suggestions = new Suggestions();
-        this._suggestions.connect('suggestion-clicked', (suggestions, str) => {
-            this._keyboardController.commitString(str);
-        });
         this.actor.add(this._suggestions.actor,
                        { x_align: St.Align.MIDDLE,
                          x_fill: false });