added delete function to th application

controllers/delete.php

@@ -0,0 +1,12 @@
+<?php
+$heading = "Delete";
+$username = "MatheApp";
+$password = "password";
+$config = require('config.php');
+$db = new Database($config['database'], $username, $password);
+
+session_start();
+$db->query("DELETE FROM user WHERE username = :user", ["user" => $_SESSION['username']]);
+session_destroy();
+header("Location: /");
+exit;