Move model definition files from `models/thing/thing.js` to
`models/thing.js` and remove the now empty intermediate directory.
Task-2759582
* = calendar, crm_livechat, hr, im_livechat, mail_bot, sms, snailmail,
website_livechat, website_slides
closes odoo/odoo#86464
Related: odoo/enterprise#25291
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>