zilencer: Drop the index from RemotePushDeviceToken.user_id.

The index isn't used, because our unique_index entries provide better
indexes for the queries.

zilencer/migrations/0025_alter_remotepushdevicetoken_user_id_drop_index.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.12 on 2022-03-10 12:34
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zilencer", "0024_remotepushdevicetoken_user_uuid"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="remotepushdevicetoken",
+            name="user_id",
+            field=models.BigIntegerField(null=True),
+        ),
+    ]