Remove http_io_previous_chunk_cache feature flag

What does this MR do and why?

Removes the http_io_previous_chunk_cache feature flag introduced in 19.3 and makes the previous-chunk caching behaviour in Gitlab::HttpIO unconditional.

The flag was a gitlab_com_derisk type used to de-risk the rollout of retaining the previously read 128 KB chunk so that backward reads straddling a chunk boundary do not re-request chunks they just evicted. The rollout is complete; the behaviour is now always enabled.

Changes:

• Delete config/feature_flags/gitlab_com_derisk/http_io_previous_chunk_cache.yml
• Remove the @cache_previous_chunk instance variable and its Feature.enabled? check from Gitlab::HttpIO#initialize
• Remove the return false unless @cache_previous_chunk guard from restore_previous_chunk and the return unless @cache_previous_chunk guard from retain_current_chunk
• Remove the feature-flag-specific test contexts from spec/lib/gitlab/http_io_spec.rb

References

• Rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/608094
• Introducing MR: !248610 (merged)

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