Merge branch '213416-remove-unused-background-migration' into 'master'

Remove unused LinkLfsObjects BG migration

Closes #213416

See merge request gitlab-org/gitlab!28886

lib/gitlab/background_migration/link_lfs_objects.rb

-# frozen_string_literal: true
-
-module Gitlab
-  module BackgroundMigration
-    # Create missing LfsObjectsProject records for forks
-    class LinkLfsObjects
-      # Model definition used for migration
-      class ForkNetworkMember < ActiveRecord::Base
-        self.table_name = 'fork_network_members'
-
-        def self.with_non_existing_lfs_objects
-          joins('JOIN lfs_objects_projects lop ON fork_network_members.forked_from_project_id = lop.project_id')
-            .where(
-              <<~SQL
-                NOT EXISTS (
-                  SELECT 1
-                  FROM lfs_objects_projects
-                  WHERE lfs_objects_projects.project_id = fork_network_members.project_id
-                  AND lfs_objects_projects.lfs_object_id = lop.lfs_object_id
-                )
-              SQL
-            )
-        end
-      end
-
-      def perform(start_id, end_id)
-        # no-op as some queries times out
-      end
-    end
-  end
-end