fixes #3575

Closes #3575

Merge request studip/studip!2469

app/views/consultation/admin/cancel_slots.php

 <form action="<?= $action ?>" method="post" class="default">
     <?= CSRFProtection::tokenTag() ?>
+    <input type="hidden" name="cancel" value="1">
 
     <?= addHiddenFields('slot-id', $slots->map(function ($slot) {
         return "{$slot->block_id}-{$slot->id}";