Fix Sidekiq NameError for BackfillNotificationJobsWorker

What does this MR do and why?

These workers were removed with !129957 (merged), but their cron job definitions still exist. This means that when sidekiq tries to run the cron jobs, it encounters an uninitialized constant exception.

2023-09-13_15:16:49.73178 rails-background-jobs : {"severity":"WARN","time":"2023-09-13T15:16:49.731Z","retry":25,"queue":"default","backtrace":true,"class":"Namespaces::FreeUserCap::BackfillNotificationJobsWorker","args":[],"jid":"159a9338c61e519f1069ff3c","created_at":"2023-09-13T15:15:05.001Z","correlation_id":"c6c295934c9de83f69518d241c446a06","worker_data_consistency":"always","idempotency_key":"resque:gitlab:duplicate:default:15a9dd13375ceca3a830d4859283c154f146e68e4064592bf46740c2f79c0ef6","duplicate-of":"e072dae2a27d8d08ff50e444","size_limiter":"validated","enqueued_at":"2023-09-13T15:16:49.674Z","retry_count":2,"retried_at":"2023-09-13T15:16:05.412Z","job_size_bytes":2,"pid":38087,"message":"Namespaces::FreeUserCap::BackfillNotificationJobsWorker JID-159a9338c61e519f1069ff3c: fail: 0.002199 sec","job_status":"fail","scheduling_latency_s":0.000749,"duration_s":0.002199,"completed_at":"2023-09-13T15:16:49.677Z","exception.class":"NameError","exception.message":"uninitialized constant Namespaces::FreeUserCap::BackfillNotificationJobsWorker\n\n        constant.const_get(name, false)\n                ^^^^^^^^^^","exception.backtrace":["lib/gitlab/sidekiq_logging/structured_logger.rb:21:in `call'"],"db_duration_s":0.0}

Remove the cron job definitions since the workers no longer exist.

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