Fix: remove non-existent cancelled_at from enrollments update

The enrollments table has no cancelled_at column — only update status.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Livewire/Participants/DeleteParticipant.php

@@ -110,7 +110,7 @@ public function deleteParticipant(): void
         DB::transaction(function () use ($participant) {
             $participant->enrollments()
                 ->whereIn('status', ['active', 'pending', 'waitlisted'])
-                ->update(['status' => 'cancelled', 'cancelled_at' => now()]);
+                ->update(['status' => 'cancelled']);
 
             AttendanceRecord::where('subject_type', Participant::class)
                 ->where('subject_id', $participant->id)