[FIX] portal: remove useless dependency to 'web_editor.ready'

Following the new editor's merge at https://github.com/odoo/odoo/pull/29775,
the 'web_editor.ready' module was moved to website without caring about
where it was used.

Fortunately the only non-website app which uses it is portal in a module
which in fact does not need it.
It would probably make more sense if it was in portal though (see
https://github.com/odoo/odoo/pull/30409) but as an upcoming PR is about
to fix those problems (https://github.com/odoo/odoo/pull/29442), this
can stay in website for now.

closes odoo/odoo#30473

addons/portal/static/src/js/portal_signature.js

@@ -1,8 +1,6 @@
 odoo.define('portal.signature_form', function (require){
     "use strict";
 
-    require('web_editor.ready');
-
     var ajax = require('web.ajax');
     var base = require('web_editor.base');
     var core = require('web.core');