Comment out migration helper include in migration template

It is common for database reviewers to request this
include be removed if none of the helpers are used,
this change is an effort to reduce these comments by
making this include opt-in instead of opt-out.

generator_templates/active_record/migration/migration.rb

@@ -4,7 +4,8 @@
 # for more information on how to write migrations for GitLab.
 
 class <%= migration_class_name %> < ActiveRecord::Migration[<%= ActiveRecord::Migration.current_version %>]
-  include Gitlab::Database::MigrationHelpers
+  # Uncomment the following include if you require helper functions:
+  # include Gitlab::Database::MigrationHelpers
 
   # Set this constant to true if this migration requires downtime.
   DOWNTIME = false

generator_templates/rails/post_deployment_migration/migration.rb

@@ -4,7 +4,8 @@
 # for more information on how to write migrations for GitLab.
 
 class <%= migration_class_name %> < ActiveRecord::Migration[<%= ActiveRecord::Migration.current_version %>]
-  include Gitlab::Database::MigrationHelpers
+  # Uncomment the following include if you require helper functions:
+  # include Gitlab::Database::MigrationHelpers
 
   DOWNTIME = false