Only require sidekiq-limit_fetch when enabled in settings

What does this MR do?

To test this out, run gdk redis-cli monitor | grep limit_fetch:

1. Before this change, there is a lot of output (all misses), even when throttling is disabled.
2. After this change, there is no output when throttling is disabled.
3. Enabling throttling and restarting Sidekiq still does the throttling.

Are there points in the code the reviewer needs to double check?

Don't think so - we already needed people to restart for this to take effect.

Why was this MR needed?

Just having this gem included created a lot of cache misses.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/36253.