[FIX] hr_holidays: do not allow reduce remaining leaves

In the Employee form, a remaining legal leaves field is available, which shows remaining validated leaves from allocation requests
As this is not allowed to delete a validated allocation requets, it shouldn't be possible to reduce the validated remaining leaves of an employee.
To reduce the remaining leaves of an employee, the user should cancel and remove the allocation request
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32