Refine task form title to avoid duplication

Remove redundant "Modifier la tâche" heading in the task editing form.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: cc2d271b-82f0-4d6a-9302-a0b067ffb429
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/1957c339-2757-4d1f-8e92-e9f71a1ce58e/cc2d271b-82f0-4d6a-9302-a0b067ffb429/gKbtsJQ

client/src/components/tasks/TaskFormClean.tsx

@@ -174,9 +174,6 @@ export default function TaskForm({ task, onClose }: TaskFormProps) {
     <div className="space-y-6">
       <div className="flex items-center justify-between border-b pb-4">
         <div>
-          <h2 className="text-xl font-semibold">
-            {task ? "Modifier la tâche" : "Nouvelle tâche"}
-          </h2>
           <p className="text-sm text-muted-foreground">
             📅 Date de début = Quand la tâche devient visible<br/>
             ⏰ Date d'échéance = Quand la tâche doit être terminée