settings: Remove private_message_policy setting.

Removes `private_message_policy` field from both the API and
the backend.

zerver/migrations/0552_remove_realm_private_message_policy.py

@@ -0,0 +1,16 @@
+# Generated by Django 5.0.6 on 2024-06-27 04:46
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0551_alter_realm_direct_message_initiator_group_and_more"),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name="realm",
+            name="private_message_policy",
+        ),
+    ]