Remove 'single_price' field from supplier part

- Instead we will rely entirely on the SupplierPriceBreak model

InvenTree/part/migrations/0026_remove_supplierpart_single_price.py

@@ -0,0 +1,17 @@
+# Generated by Django 2.2 on 2019-05-18 05:43
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0025_auto_20190515_0012'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='supplierpart',
+            name='single_price',
+        ),
+    ]