Move CleanupMultiprocDirService out of config.ru

What does this MR do?

Implements #300652 (closed).

Originally, cleanup in config.ru was introduced in gitlab-foss!31668 (merged), but should be no longer necessary.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

What to test:

• [omni/cluster] run gitlab-ctl hup puma
• [omni/cluster] run gitlab-ctl hup sidekiq
• [omni/cluster] run gitlab-ctl restart puma
• [omni/cluster] run gitlab-ctl restart sidekiq
• [cng/cluster] check that metrics are present on CNG after the boot - my Minikube setup got broken, it seems. Need to spend some time figuring out why it's not working or find a workaround.
• [omni] ensure the dedicated metrics endpoint works (not /-/metrics): curl http://127.0.0.1:9168/metrics | wc -l => 1218
• [gdk/cluster] check metrics are present - ⚠ for some reason, don't see sidekiq on GDK, both on master and on this branch. Probably not related, sidekiq is present on GCK (Omnibus). I run ruby_process_cpu_seconds_total.
• [gdk/single] check metrics are present
• [omni/cluster] check that lib/gitlab/metrics/requests_rack_middleware.rb metrics are not stuck
• [omni/single] check that Puma Single mode has metrics now
• please ping me if you have any other concerns!

Screenshots

Metrics in Puma Single are present now.

N.B.: we'll likely rename puma_master into just puma for a Single Mode, but I'd prefer to do that in a separate MR.

Security

N/A

Related to #300652 (closed)