Before the commit, `_action_unfollow()` can be called by
`_message_receive_bounce()` and then multiple leave notifications
can be sent even if the partner has been removed from the channel,
when the partner is a follower of the channel and without a
valid email address.
Unfollow action should also remove the partner from the followers,
and only be processed if the partner is still a member of the channel.
Task id: 2456233
closesodoo/odoo#68314
X-original-commit: 5cf22425eb68f15d94a8b283c50198561bbc240a
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>