web/web_x2many_delete_all/templates/assets.xml

<?xml version="1.0" encoding="UTF-8"?>
<odoo>
    <template id="assets_backend" inherit_id="web.assets_backend">
        <xpath expr=".">
            <script type="text/javascript"
                    src="/web_x2many_delete_all/static/src/js/web_x2many_delete_all.js"/>
        </xpath>
    </template>
</odoo>