Remove defunct 'build_order' field from StockItem model

- This is now handled by the new-and-improved build system, no longer required

InvenTree/build/migrations/0023_auto_20201110_0911.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.0.7 on 2020-11-10 09:11
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0054_remove_stockitem_build_order'),
+        ('build', '0022_buildorderattachment'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='builditem',
+            name='stock_item',
+            field=models.ForeignKey(help_text='Source stock item', limit_choices_to={'belongs_to': None, 'sales_order': None}, on_delete=django.db.models.deletion.CASCADE, related_name='allocations', to='stock.StockItem'),
+        ),
+    ]