Always create indexing tasks for empty zoekt repositories

What does this MR do and why?

Always create indexing tasks for empty zoekt repositories. This releases the feature flag zoekt_index_empty_repos.

AI Summary

This change removes a feature flag called zoekt_index_empty_repos and simplifies the code that handles indexing of empty repositories in the search system.

Previously, the system had conditional logic that would either index empty repositories or skip them based on whether this feature flag was enabled. Now, the code has been streamlined to always handle empty repositories the same way - it will skip indexing them but still process deletion tasks when needed.

The changes remove the feature flag configuration file, eliminate the should_create_indexing_task? method that checked this flag, and simplify several service methods to have more straightforward behavior. All the related test cases that were testing the different behaviors when the flag was on or off have also been removed.

Essentially, this is a cleanup that removes experimental functionality and makes the search indexing behavior more consistent and predictable by removing the toggle that allowed different handling of empty repositories.

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