Remove store_arkose_session feature flag

Ruby Nealonrequested to merge
rubynealon/remove-store-arkose into master

What does this MR do and why?

Remove store_arkose_session feature flag

Remove the store_arkose_session feature flag and update docs to reflect the feature being enabled on GitLab.com.

References

https://gitlab.com/gitlab-org/gitlab/-/issues/549743

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