Remove queue_size method from ApplicationWorker

What does this MR do and why?

Related to gitlab-com/gl-infra/scalability#1936 (closed)

This MR removes queue_size from ApplicationWorker and prevent sidekiq workers from accessing queue-size. This is to prepare for zonal sidekiq which shards our sidekiq redis, making queue-length harder to reason about (do workers want to know global queue size/sum or their zonal size, etc).

With !98171 (merged), queue_size is no longer used.

It is also part of initial ideas for a framework when working on sidekiq-workers, queue system's state should not be known to workers. See gitlab-com/gl-infra/scalability#1453 (comment 1088858196)

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