[IMP] hr_holiday: remove leave onchange date check

When you create a leave, you first set the start date. The date widget triggers
and automatically sets you at today, which triggers the onchange and puts the
end date at today + 8 hours. You then set your start date in the future
-> invalid dates because start date > end date.

This commit removes the onchange verification of the dates, because almost
everyone gets the above problem and because it is redundant with the sql
constraint that triggers anyway when you try to save with invalid dates.