Resolve "Remove MergeRequests::Llm::SummarizeMergeRequestWorker"

What does this MR do and why?

With the release of the feature flag (#423209 (closed) and !137409 (merged)) we are now not using this worker since 2 minor releases ago and have no-oped it since 1 release. This worker was originally used for the /summarize quick action, and then used for the first iteration of the automatic summarize. Now we use the generic LLM worker to achieve this.

Following the compatibility guide: https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html

Kibana search for the worker: https://log.gprd.gitlab.net/app/r/s/2Wlbr

Related Issue: #423210 (closed)