Remove the default value for the `users.ghost` database column.

The default (false) is not strictly required, and this lets us avoid a
potentially expensive migration

db/migrate/20170206115204_add_column_ghost_to_users.rb

 class AddColumnGhostToUsers < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
   DOWNTIME = false
 
-  disable_ddl_transaction!
-
   def up
-    add_column_with_default :users, :ghost, :boolean, default: false, allow_null: false
+    add_column :users, :ghost, :boolean
   end
 
   def down

db/schema.rb

@@ -1278,11 +1278,11 @@ ActiveRecord::Schema.define(version: 20170216141440) do
     t.datetime "otp_grace_period_started_at"
     t.boolean "ldap_email", default: false, null: false
     t.boolean "external", default: false
-    t.string "organization"
     t.string "incoming_email_token"
+    t.string "organization"
     t.boolean "authorized_projects_populated"
     t.boolean "notified_of_own_activity", default: false, null: false
-    t.boolean "ghost", default: false, null: false
+    t.boolean "ghost"
   end
 
   add_index "users", ["admin"], name: "index_users_on_admin", using: :btree