The `member_values` argument of `action_add_member` is no
longer used in any call of `action_add_member`. We can therefore
remove it in standard, as it's completely unused.
Besides, after grepping the code for all calls to `action_add_member`
to check `member_values` was indeed unused, I saw this method
wasn't used by any views or Javascript code, but only
through Python routes, and therefore it can be converted
to a private method by prefixing it with `_`,
as our guidelines states to make private methods which are not used
by the web client.
closesodoo/odoo#111908
Signed-off-by: Denis Ledoux (dle) <dle@odoo.com>