analytics: Remove last_modified field from FillState.

This field wasn't used for anything, and I think it has very limited
use for debugging, since fundamentally, it'll almost always have a
value within the hour of the actual timestamp in FillState, and any
more fine-grained logging we might want would be available in the
analytics job's own logs.

The proximal reason to remove it is that apparently Django's
model_to_dict doesn't support auto_now fields, and that caused some
trouble when working on adding more complete import/export support for
analytics data.

analytics/migrations/0014_remove_fillstate_last_modified.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.26 on 2020-01-27 04:32
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('analytics', '0013_remove_anomaly'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='fillstate',
+            name='last_modified',
+        ),
+    ]

analytics/models.py

@@ -15,8 +15,6 @@ class FillState(models.Model):
     STARTED = 2
     state = models.PositiveSmallIntegerField()  # type: int
 
-    last_modified = models.DateTimeField(auto_now=True)  # type: datetime.datetime
-
     def __str__(self) -> str:
         return "<FillState: %s %s %s>" % (self.property, self.end_time, self.state)