Remove backfill migration for legacy projects

We've found some problems with the backgroung migration:
BackfillLegacyProjectRepositories

See https://gitlab.com/gitlab-org/gitlab-ce/issues/56061

The migration was disabled to run on production:
https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5906

So to overcome it's not being run anywhere, just remove it.

Whenever all problems in BackfillLegacyProjectRepositories are fixed
we'll create a new migration (with a different timestamp) to have it
run.

changelogs/unreleased/tc-remove-20181218192239-migration.yml

+---
+title: Remove migration to backfill project_repositories for legacy storage projects
+merge_request: 24299
+author:
+type: removed

db/post_migrate/20181218192239_backfill_project_repositories_for_legacy_storage_projects.rb

-# frozen_string_literal: true
-
-class BackfillProjectRepositoriesForLegacyStorageProjects < ActiveRecord::Migration[5.0]
-  include Gitlab::Database::MigrationHelpers
-
-  DOWNTIME       = false
-  BATCH_SIZE     = 1_000
-  DELAY_INTERVAL = 5.minutes
-  MIGRATION      = 'BackfillLegacyProjectRepositories'
-
-  disable_ddl_transaction!
-
-  class Project < ActiveRecord::Base
-    include EachBatch
-
-    self.table_name = 'projects'
-  end
-
-  def up
-    queue_background_migration_jobs_by_range_at_intervals(Project, MIGRATION, DELAY_INTERVAL)
-  end
-
-  def down
-    # no-op: since there could have been existing rows before the migration do not remove anything
-  end
-end

spec/lib/gitlab/background_migration/backfill_legacy_project_repositories_spec.rb

@@ -2,6 +2,6 @@
 
 require 'spec_helper'
 
-describe Gitlab::BackgroundMigration::BackfillLegacyProjectRepositories, :migration, schema: 20181218192239 do
+describe Gitlab::BackgroundMigration::BackfillLegacyProjectRepositories, :migration, schema: 20181212171634 do
   it_behaves_like 'backfill migration for project repositories', :legacy
 end