Remove search_zoekt_task_processing_queue_size metric reporting

Background

#521933 (closed) identified the prometheus metrics from cron workers as not accurate.

Mark zoekt prometheus metric as deprecated (!184641 - merged) has marked the search_zoekt_task_processing_queue_size as deprecated for removal in 18.0, but this metric is for a feature in beta so it can be removed earlier.

The metric will now be emitted in gitlab-exporter, see gitlab-org/ruby/gems/gitlab-exporter!216 (merged)

Proposal

Remove the code and documentation for search_zoekt_task_processing_queue_size metric