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(),
+        ),
+    ]

InvenTree/stock/models.py

@@ -292,11 +292,6 @@ class StockItem(MPTTModel):
     def get_part_name(self):
         return self.part.full_name
 
-    class Meta:
-        unique_together = [
-            ('part', 'serial'),
-        ]
-
     def format_barcode(self):
         """ Return a JSON string for formatting a barcode for this StockItem.
         Can be used to perform lookup of a stockitem using barcode