Remove defunct 'build_order' field from StockItem model

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

InvenTree/stock/migrations/0054_remove_stockitem_build_order.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.0.7 on 2020-11-10 09:11
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0053_auto_20201110_0513'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='stockitem',
+            name='build_order',
+        ),
+    ]