Remove use of multi in issues rebalancing state

What does this MR do and why?

This MR simplifies the cleanup step to be robust against Redis errors and job retry. The main goal is for Gitlab::Issues::Rebalancing::State's Redis operations to be compatible with Redis Cluster.

See discussion in #416438 (comment 1466586535) on correctness

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.