[controller] Stop writing writing ready-to-serve instances info in meta system store

[sushantmane]

Stop writing writing ready-to-serve instances info in meta system store

When materializing the meta store, we produce a snapshot of readyToServeInstances for all user store versions. If a version is in an ERROR state, its Helix resources may not exist. Attempting to check these resources can throw exceptions, potentially causing the addVersion process to fail.

The info about RTS instances in meta store was required in the past but it is no
longer used by any clients so it's safe to stop writing this into meta system store.

How was this PR tested?

Does this PR introduce any user-facing changes?

• [x] No. You can skip the rest of this section.
• [ ] Yes. Make sure to explain your proposed changes and call out the behavior change.

[sushantmane]

Thanks for the review, @sixpluszero!