[FIX] im_livechat, website_livechat: fix use visitor name in livechat session

im_livechat main controller was referring to website.visitor but this module is
not depending on website. The usage of website visitor's name should have been
done in website_livechat module.

This commit fix this issue.

Task ID: 2116715
PR #40732

closes odoo/odoo#40884

X-original-commit: defb98c1
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>