Fix error 2800 when deleting custom field

This error is a regression introduced by fix for issue 0027274.

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 0033248