lib: Remove form element `Note'

Zend already provides this.

library/Icinga/Web/Form/Element/Note.php

@@ -1,31 +0,0 @@
-<?php
-// {{{ICINGA_LICENSE_HEADER}}}
-// {{{ICINGA_LICENSE_HEADER}}}
-
-namespace Icinga\Web\Form\Element;
-
-use Zend_Form_Element_Xhtml;
-
-/**
- * Implements note element for Zend forms
- */
-class Note extends Zend_Form_Element_Xhtml
-{
-    /**
-     * Name of the view helper
-     *
-     * @var string
-     */
-    public $helper = 'formNote';
-
-    /**
-     * Return true to ensure redrawing
-     *
-     * @param mixed $value      The value of to validate (ignored)
-     * @return bool             Always true
-     */
-    public function isValid($value)
-    {
-        return true;
-    }
-}