Remove unique-together requirement

- This is handled manually via the validate_unique function

InvenTree/stock/migrations/0044_auto_20200528_1036.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.0.5 on 2020-05-28 10:36
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0043_auto_20200525_0420'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='stockitem',
+            unique_together=set(),
+        ),
+    ]