[FIX] hr_holidays: remove allocation_id on hr.leave

As part of hr_holidays B2B, the holidays allowances were moved from hr_leave_type to hr_leave_allocation.
This didn't allow to take a leave longer than the maximum duration of an allocation. Also, despite all the
allocations could have enough days for a certain leave, it was not possible to use all available days without
splitting the leaves across the different allocations.

This commit moves the holidays allowance to the hr_leave_type to remediate the issues described above.

task-2834887

closes odoo/odoo#98317

X-original-commit: 8c37d5c7cc397e517c43c1ec13a131b4b35e48b7
Signed-off-by: phwa-odoo <phwa@odoo.com>