Drop `EmailLogs#topic_id`.

db/fixtures/000_delayed_drops.rb

@@ -239,4 +239,15 @@ Migration::ColumnDropper.drop(
   }
 )
 
+Migration::ColumnDropper.drop(
+  table: 'email_logs',
+  after_migration: 'DropTopicIdOnEmailLogs',
+  columns: %w{
+    topic_id
+  },
+  on_drop: ->() {
+    STDERR.puts "Removing superflous email_logs columns!"
+  }
+)
+
 Discourse.reset_active_record_cache

db/migrate/20180724070554_drop_topic_id_on_email_logs.rb

@@ -0,0 +1,5 @@
+class DropTopicIdOnEmailLogs < ActiveRecord::Migration[5.2]
+  def change
+    remove_index :email_logs, :topic_id
+  end
+end