[FIX] web_editor, mass_mailing: speed up opening of field html inline

On opening field html with the inline styles option we were parsing all
the css in the page. This should only be needed when we save. This
commit moves the call to `getCSSRules`back to `commitChanges`.

closes #83074

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/mass_mailing/static/src/js/mass_mailing_widget.js

@@ -115,10 +115,6 @@ var MassMailingFieldHtml = FieldHtml.extend({
         this.$content.find('.o_layout').addBack().data('name', 'Mailing');
         // We don't want to drop snippets directly within the wysiwyg.
         this.$content.removeClass('o_editable');
-        // Force compute CSS rules even without the style-inline node option.
-        if (this.mode === 'edit' && !this.cssRules) {
-            this.cssRules = convertInline.getCSSRules(this.wysiwyg.getEditable()[0].ownerDocument);
-        }
     },
     /**
      * Returns true if the editable area is empty.

addons/web_editor/static/src/js/backend/convert_inline.js

@@ -955,14 +955,6 @@ FieldHtml.include({
     // Public
     //--------------------------------------------------------------------------
 
-    _createWysiwygIntance: function () {
-        return this._super(...arguments).then(() => {
-            if (this.nodeOptions['style-inline'] && this.mode === "edit") {
-                this.cssRules = getCSSRules(this.wysiwyg.getEditable()[0].ownerDocument);
-            }
-        });
-    },
-
     /**
      * @override
      */