[FIX] hr: Remove unnecessary argument

Followup on 5ce8f1e3
Before this commit, There would be traceback on Launching Plan with `Responsible` set to `other` and No `Responsible Person` defined on activity due to Extra argument.

In this commit, We remove that extra Argument.

closes odoo/odoo#44965

X-original-commit: 009e5526
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/hr/models/hr_plan.py

@@ -50,7 +50,7 @@ class HrPlanActivityType(models.Model):
         elif self.responsible == 'other':
             responsible = self.responsible_id
             if not responsible:
-                raise UserError(_('No specific user given on activity.') % employee.name)
+                raise UserError(_('No specific user given on activity.'))
         return responsible