Remove global_search_custom_slis feature flag

Roy Zwambagrequested to merge
rz-remove-global_search_custom_slis-ff into master

What does this MR do and why?

This feature flag has been on for over two weeks now and is running stable, the feature is completely integrated and we can remove this feature flag.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #372107 (closed)

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