remove migration to index impressionable_type, message, and impressionable_id

upgrade_migrations/version_1_1_2.rb

@@ -1,9 +0,0 @@
-class Version04UpdateImpressionsTable < ActiveRecord::Migration
-  def self.up
-    add_index :impressions, [:impressionable_type, :message, :impressionable_id], :name => "impressionable_type_message_index", :unique => false
-  end
-
-  def self.down
-    remove_index :impressions, :impressionable_type_message_index
-  end
-end