Fix error 2800 when deleting custom field

This error is a regression introduced by fix for issue #27274.

Changing security token name in manage_custom_field_delete.php from
manage_custom_field_delete to manage_custom_field_update resolves the
problem.

It makes more sense to reuse the existing token rather than generate a
new, specific one in manage_custom_field_edit_page.php since we're using
the same form data for update and delete operations.

Fixes #33248

manage_custom_field_delete.php

@@ -49,7 +49,7 @@
 require_api( 'print_api.php' );
 require_api( 'string_api.php' );
 
-form_security_validate( 'manage_custom_field_delete' );
+form_security_validate( 'manage_custom_field_update' );
 
 auth_reauthenticate();
 access_ensure_global_level( config_get( 'manage_custom_fields_threshold' ) );
@@ -71,6 +71,6 @@
 
 custom_field_destroy( $f_field_id );
 
-form_security_purge( 'manage_custom_field_delete' );
+form_security_purge( 'manage_custom_field_update' );
 
 print_header_redirect( $f_return );