remove index 'email' from register_tokens

only records with the same UUID are deleted before INSERTing new ones
meaning a player could prevent another one from using a certain address
when emails are unique.

There's no good reason to force uniqueness on emails in this table

db/migrate/20160926220738_remove_index_email_from_register_tokens.rb

@@ -0,0 +1,5 @@
+class RemoveIndexEmailFromRegisterTokens < ActiveRecord::Migration
+  def change
+    remove_index :register_tokens, :email
+  end
+end