Drop Iglu registry URL column

Second step for 3-release column removal

changelogs/unreleased/244828-drop-iglu-registry-url-column.yml

+---
+title: Drop Iglu registry URL column
+merge_request: 42939
+author:
+type: removed

db/post_migrate/20200922054642_drop_snowplow_iglu_registry_url_from_application_settings.rb

+# frozen_string_literal: true
+
+class DropSnowplowIgluRegistryUrlFromApplicationSettings < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def change
+    remove_column :application_settings, :snowplow_iglu_registry_url, :string, limit: 255
+  end
+end

db/schema_migrations/20200922054642

+ad63096e49440f7f2a15ea2747689ca39f52fdcebc1949a1feed82a22f432e9e
\ No newline at end of file

db/structure.sql

@@ -9196,7 +9196,6 @@ CREATE TABLE application_settings (
     throttle_incident_management_notification_enabled boolean DEFAULT false NOT NULL,
     throttle_incident_management_notification_period_in_seconds integer DEFAULT 3600,
     throttle_incident_management_notification_per_period integer DEFAULT 3600,
-    snowplow_iglu_registry_url character varying(255),
     push_event_hooks_limit integer DEFAULT 3 NOT NULL,
     push_event_activities_limit integer DEFAULT 3 NOT NULL,
     custom_http_clone_url_root character varying(511),