[FIX] mrp: successively plan/unplan the same mo

Introduces by #0679d0d5 when you plan a
workorder, the corresponding workcenter will reserve a leave to avoid to
plan an another workorder in the same time slot.
But before this commit, when you unplan your workorder, the leave wasn't
unlinked, so the time slot stay reserved.
Now when a workorder is unlinked, its leave is too.

closes odoo/odoo#35003

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>