[fix] Fixed migrations for sample app

Author: pandafy

tests/openwisp2/sample_radius/migrations/0032_registered_user_multitenant.py

@@ -192,7 +192,6 @@ class Migration(migrations.Migration):
                         help_text=REGISTERED_USER_ORGANIZATION_HELP_TEXT,
                         null=True,
                         on_delete=django.db.models.deletion.CASCADE,
-                        related_name="registered_users",
                         to=swapper.get_model_name("openwisp_users", "Organization"),
                         verbose_name="organization",
                     ),