[TASK] Remove unused FlashMessageController

ex:backend `FlashMessageController` and its route
identifier `ajax_flashmessages_render` with route
path `ajax/flashmessages/render` is internal, unused
and removed with the patch.

Resolves: #105999
Releases: main
Change-Id: I9ef7cf83b079a36361bdd4f239c8e25ef7c0732b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/87918
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Andreas Kienast <akienast@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Andreas Kienast <akienast@scripting-base.de>

Author: lolli42

typo3/sysext/backend/Classes/Controller/FlashMessageController.php

@@ -272,12 +272,6 @@
         'target' => Controller\MfaAjaxController::class . '::handleRequest',
     ],
 
-    // Render flash messages
-    'flashmessages_render' => [
-        'path' => '/flashmessages/render',
-        'target' => \TYPO3\CMS\Backend\Controller\FlashMessageController::class . '::getQueuedFlashMessagesAction',
-    ],
-
     // Load context menu for
     'contextmenu' => [
         'path' => '/context-menu',