Cleanup leftovers in packages_dependencies table

What does this MR do and why?

MR !20549 (merged) added two tables: packages_dependencies and packages_dependency_links. When a package is deleted, packages_dependency_links rows are properly deleted but rows in packages_dependencies are kept and they can become orphans. This MR aims to clean up those orphaned packages_dependencies records.

How to set up and validate locally

1. In Rails console, create a bunch of orphaned packages_dependencies records: FactoryBot.create_list(:packages_dependency, 100)
2. Enable the feature flag : Feature.enable(:packages_delete_orphaned_dependencies_worker)
3. Run the cleanup sidekiq worker: Packages::Cleanup::DeleteOrphanedDependenciesWorker.perform_async, the orphaned records should be deleted.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Database analysis

1. !113076 (comment 1297596269)
2. !113076 (comment 1322050386)

Related to #38266 (closed)

app/workers/packages/cleanup/delete_orphaned_dependencies_worker.rb

@@ -5,7 +5,6 @@ module Cleanup
     class DeleteOrphanedDependenciesWorker
       include ApplicationWorker
       include LimitedCapacity::Worker
-      include Gitlab::Utils::StrongMemoize
 
       data_consistency :always
       queue_namespace :package_cleanup
@@ -25,7 +24,6 @@ def perform_work
       def remaining_work_count
         orphaned_dependencies.limit(max_running_jobs + 1).count
       end
-      strong_memoize_attr :remaining_work_count
 
       def max_running_jobs
         MAX_CAPACITY