[REV] mass_mailing, website: simplify s_numbers snippet

This reverts commit [1] (in master only since XML change) which was a
temporary fix. Indeed, the proper fix was merged at [2] since then.

[1]: https://github.com/odoo/odoo/commit/886af1f328a9f1664b0d3e204bf984bfee28b215
[2]: https://github.com/odoo/odoo/commit/5e3c466e1b6a23a76807ab4f4d81bcf70105eedb



closes odoo/odoo#86935

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>