[REV] crm_livechat: Revert "[FIX] crm_livechat : prevent lead without stage"

This reverts commit odoo/odoo@b3f67772dd029c0349302970315ece420f44340c. Inded this implies functional
changes linked to lead assignment. Moreover this is more linked to specific
configuration than a bugfix. Indeed having shared unfolded stage fixes the
issue (team_id=False, fold=False on stage).

closes odoo/odoo#73946

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