🗃️(models) remove 'secret' field from mailbox model

We remove 'secret' field, as it won't be of use in interactions
between la Régie and dimail. Régie credentials will be stored and used
as project variable.

src/backend/mailbox_manager/migrations/0013_remove_maildomain_secret.py

@@ -0,0 +1,17 @@
+# Generated by Django 5.1 on 2024-08-30 12:46
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('mailbox_manager', '0012_alter_mailbox_local_part'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='maildomain',
+            name='secret',
+        ),
+    ]