Delete Unused Test Member

tests/PhpSpreadsheetTests/Shared/DateMalformedStringException.php

@@ -1,20 +0,0 @@
-<?php
-
-/**
- * For the full copyright and license information, please view the
- * LICENSE file that was distributed with this source code.
- */
-
-declare(strict_types=1);
-
-namespace {
-    if (!class_exists(DateMalformedStringException::class)) {
-        /**
-         * Class DateMalformedStringException
-         * Polyfill class for the DateMalformedStringException.
-         */
-        class DateMalformedStringException extends Exception
-        {
-        }
-    }
-}