Delete Translation.source_last_updated_at

This field isn't used, or updated. It was accidentally left over from a
previous refactoring.

Users should use ``Translation.source.last_updated_at`` instead.

wagtail_localize/migrations/0014_remove_translation_source_last_updated_at.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.2.6 on 2021-09-14 08:53
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtail_localize', '0013_translationsource_schema_version'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='translation',
+            name='source_last_updated_at',
+        ),
+    ]

wagtail_localize/models.py

@@ -898,7 +898,6 @@ class Translation(models.Model):
     )
 
     created_at = models.DateTimeField(auto_now_add=True)
-    source_last_updated_at = models.DateTimeField(auto_now_add=True)
     translations_last_updated_at = models.DateTimeField(null=True)
     destination_last_updated_at = models.DateTimeField(null=True)
     enabled = models.BooleanField(default=True)