The goal of commit 2dbce5e6 was to handle the company
holiday type in onchange which was probably forgotten
when the company type was introduced.
However because the last `elif` was changed to `else`,
the `else` clause is executed in some situations where
it shouldn't.

e.g. If holiday type is `employee` and an employee is set,
the `else` clause is (obviously wrongly) executed.
This is particularly a problem if the employee is not
HR user nor manager. In that case, the field employee_id
is invisible and its value has been deleted by the onchange.
The allocation cannot be saved because it violates the
sql constraint (employee is null).

Also, the `mode_company_id` and `category_id` should be set
to `False` when the holiday type is not company/category.