Fix: remove be_main extend for Contao 5 compatibility

contao/templates/be_dummy_copier.html5

@@ -1,6 +1,3 @@
-<?php $this->extend('be_main'); ?>
-
-<?php $this->block('main'); ?>
 <form action="<?= $this->action; ?>" method="post" style="max-width:900px;">
     <input type="hidden" name="REQUEST_TOKEN" value="<?= $this->requestToken; ?>">
     <input type="hidden" name="FORM_SUBMIT" value="tl_dummy_copier">
@@ -171,4 +168,3 @@
       })();
     </script>
 </form>
-<?php $this->endblock(); ?>