revert db migration

db/migrate/20160130191533_create_topic_relationships.rb

+class CreateTopicRelationships < ActiveRecord::Migration
+  def change
+    create_table :topic_relationships do |t|
+      t.references :parent, index: true
+      t.references :child, index: true
+
+      t.timestamps null: false
+    end
+  end
+end