[FIX] hr_holidays: Impossible to delete a refused Time-Off request

Steps to reproduce the bug:

-Create a time-Off request R1 for 27/01
-Refuse R1
-Create another Time-Off request R2 for the same date 27/01
-Set R1 in draft
-Try to delete R1

Bug:

A ValidationError was raised:

You can not set 2 time off that overlaps on the same day for the same employee.

opw:2447540

closes odoo/odoo#65077

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>