fix: remove unapplicable conditional unique constraint on subsidy

ENT-7891 | MySQL has never supported conditional unique constraints.
This commit removes a conditional unique constraint declared
in the subsidy model's Meta class.  It also has a no-op migration for same.
Lastly, adds a migration for historical subsidy records from
upgraded django-simple-history.

enterprise_subsidy/apps/subsidy/migrations/0019_alter_historicalsubsidy_options.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.2.19 on 2023-10-31 13:02
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('subsidy', '0018_alter_historicalsubsidy_options'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='historicalsubsidy',
+            options={'get_latest_by': 'history_date', 'ordering': ('-history_date', '-history_id'), 'verbose_name': 'historical subsidy'},
+        ),
+    ]

enterprise_subsidy/apps/subsidy/migrations/0020_remove_subsidy_unique_reference_id_non_internal.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.2.19 on 2023-10-31 13:04
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('subsidy', '0019_alter_historicalsubsidy_options'),
+    ]
+
+    operations = [
+        migrations.RemoveConstraint(
+            model_name='subsidy',
+            name='unique_reference_id_non_internal',
+        ),
+    ]

enterprise_subsidy/apps/subsidy/models.py

@@ -104,13 +104,6 @@ class Meta:
         """
         Metaclass for Subsidy.
         """
-        constraints = [
-            models.UniqueConstraint(
-                condition=Q(internal_only=False),  # Allow more flexibility for internal/test subsidies.
-                fields=["reference_id", "reference_type"],
-                name="unique_reference_id_non_internal",
-            )
-        ]
         ordering = ['-created']
 
     # Please reserve the "subsidy_type" field name for the future when we use it to distinguish between