Remove obsolete 'next_scheduled_maint' from asset model

Should fix production data...

assets/migrations/0015_remove_asset_next_sched_maint.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.0.3 on 2020-04-13 15:13
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('assets', '0014_auto_20200218_1840'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='asset',
+            name='next_sched_maint',
+        ),
+    ]