Remove :hset_redis_diff_caching feature flag

What does this MR do?

Remove code usage of the :hset_redis_diff_caching feature flag, as it was fully enabled by default in %12.7

#197469 (closed) #38008 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content