chore: DDL to remove unused threshold_type, alert_threshold and resolve_threshold fields from AlertRule (SEN-1065)

This is the ddl to actually remove the fields, once the changes to the model have been deployed.
I'll deploy this separately.

src/sentry/migrations/0016_delete_alert_rule_deprecated_fields.py

@@ -0,0 +1,43 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    # This flag is used to mark that a migration shouldn't be automatically run in
+    # production. We set this to True for operations that we think are risky and want
+    # someone from ops to run manually and monitor.
+    # General advice is that if in doubt, mark your migration as `is_dangerous`.
+    # Some things you should always mark as dangerous:
+    # - Adding indexes to large tables. These indexes should be created concurrently,
+    #   unfortunately we can't run migrations outside of a transaction until Django
+    #   1.10. So until then these should be run manually.
+    # - Large data migrations. Typically we want these to be run manually by ops so that
+    #   they can be monitored. Since data migrations will now hold a transaction open
+    #   this is even more important.
+    # - Adding columns to highly active tables, even ones that are NULL.
+    is_dangerous = False
+
+    dependencies = [("sentry", "0015_delete_sentryappwebhookerror_db")]
+
+    operations = [
+        migrations.SeparateDatabaseAndState(
+            database_operations=[
+                migrations.RunSQL(
+                    """
+                    ALTER TABLE "sentry_alertrule" DROP COLUMN "alert_threshold";
+                    ALTER TABLE "sentry_alertrule" DROP COLUMN "resolve_threshold";
+                    ALTER TABLE "sentry_alertrule" DROP COLUMN "threshold_type";
+                    """,
+                    reverse_sql="""
+                    ALTER TABLE "sentry_alertrule" ADD COLUMN "alert_threshold" smallint NULL;
+                    ALTER TABLE "sentry_alertrule" ADD COLUMN "resolve_threshold" int NULL;
+                    ALTER TABLE "sentry_alertrule" ADD COLUMN "threshold_type" int NULL;
+
+                    """,
+                )
+            ],
+            state_operations=[],
+        )
+    ]