models: Drop unique index on user/message/emoji_name.

c7d0192755 added the unique constraint on
`user_profile_id,message_id,reaction_type,emoji_code`, but left the
existing constraint on `user_profile_id,message_id,emoji_name`.  As
explained in the comment added in 3cd543ee98, `emoji_name` cannot be
trusted to be unique, as it is possible to have an Unicode emoji
reaction and a custom emoji with the same name on a message.

Remove the overly-constraining unique index, now that c7d0192755 has
provided the correct one.

zerver/migrations/0431_alter_archivedreaction_unique_together_and_more.py

@@ -0,0 +1,20 @@
+# Generated by Django 4.1.6 on 2023-02-25 01:22
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0430_fix_audit_log_objects_for_group_based_stream_settings"),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name="archivedreaction",
+            unique_together={("user_profile", "message", "reaction_type", "emoji_code")},
+        ),
+        migrations.AlterUniqueTogether(
+            name="reaction",
+            unique_together={("user_profile", "message", "reaction_type", "emoji_code")},
+        ),
+    ]

zerver/models.py

@@ -3169,10 +3169,7 @@ class AbstractEmoji(models.Model):
 class AbstractReaction(AbstractEmoji):
     class Meta:
         abstract = True
-        unique_together = (
+        unique_together = ("user_profile", "message", "reaction_type", "emoji_code")
-            ("user_profile", "message", "emoji_name"),
-            ("user_profile", "message", "reaction_type", "emoji_code"),
-        )
 
 
 class Reaction(AbstractReaction):