[IMP] mail, test_mail, hr_holidays: remove obsolete out of office

The customizable "out of office" text was removed from the editable views as
part of task-2053585, but some places in the code were still using it, even
though it was never set anymore.

This commit removes the obsolete code.

Part of task-2171864

closes odoo/odoo#44968

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>