Remove unused redis-cache feature-flag stubs

What does this MR do and why?

This MR removes feature-flag stubs in specs from missed in !127959 (merged).

Screenshots or screen recordings

Before	After

How to set up and validate locally

Run git grep for both feature flags, they should come up empty.

git grep use_primary_and_secondary_stores_for_cache
git grep use_primary_store_as_default_for_cache

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.