Remove migration guide comment:

They're accessible after doing `rails g migration` anyway.
Though I somehow feel this comment could be useful for someone
who's new and just browsing the source.

db/migrate/20160628085157_add_artifacts_size_to_ci_builds.rb

-# See http://doc.gitlab.com/ce/development/migration_style_guide.html
-# for more information on how to write migrations for GitLab.
-
 class AddArtifactsSizeToCiBuilds < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers