Change delete behaviour for parent build item

- was causing database integrity errors when a parent build existed

InvenTree/build/migrations/0024_auto_20201201_1023.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.0.7 on 2020-11-30 23:23
+
+from django.db import migrations
+import django.db.models.deletion
+import mptt.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0023_auto_20201110_0911'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='build',
+            name='parent',
+            field=mptt.fields.TreeForeignKey(blank=True, help_text='BuildOrder to which this build is allocated', null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='children', to='build.Build', verbose_name='Parent Build'),
+        ),
+    ]