Remove package metadata sync worker

Problem to solve

PackageMetadata::SyncWorker functionality was extracted and generalized as part of Add worker to trigger package metadata advisory... (#370780 - closed). But the worker itself could not be removed because of behaviour during releases (i.e. job being enqueued but the worker class no longer existed). Therefore a 2 step process must be followed as outlined here: https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#removing-worker-classes

Proposal

According to the docs, 2 steps are needed to remove a worker:

1. Make the worker's #perform method a noop.
2. Remove the worker class and remove the job from the sidekiq queue.

Step 1 was done in Add worker to trigger package metadata advisory... (#370780 - closed).

Step 2 will be done in this issue.

Implementation plan

• remove PackageMetadata::Syncworker
• regenerate queues bin/rake gitlab:sidekiq:all_queues_yml:generate
• add migration to remove job from the sidekiq queue