Remove caching_experiments feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This removes the feature flag that allowed us to disable experiment caching. After monitoring for a while and seeing that it wasn't inflating the cache in any notable ways, we feel like removing this feature flag is safe.

Additionally, if any specific experiment introduces cache bloating issues, we can immediately disable that feature flag and clean up the cache, so even in the worst case, we still have options.

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