[FIX] hr_holidays: fix multiple-employee leave

Before this commit, creating a leave for multiple employees
from the manager's page allowed to create leaves regardless
of the concerned employee's allocations.
After this commit, an error message will be displayed to inform
which employee cannot take that leave.

Also removes unused imports.

task-2995120

closes odoo/odoo#103729

X-original-commit: 636a8276e6797c1d8fe43da61d2fd26b558bfa45
Signed-off-by: Kevin Baptiste <kba@odoo.com>