[IMP]hr,hr_holidays,hr_recruitment:improve usability point of hr_onboard...

...ing task add new toggle button widget for form view.

Task: https://www.odoo.com/web#id=10564&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.zUPaUqErM8fV8bBJ

Here constraint display two same messages in warning one is from the constraint message and second from _check_holidays function which is raise the warning.
so i remove raising warning message from the function and keep only one constraint warning message.