Remove enterprise_users_automatic_claim feature flag

What does this MR do and why?

This MR removes enterprise_users_automatic_claim feature flag and all related code that was disabled by that flag.

Automatic claims of enterprise users feature has been globally enabled since Oct 20, 2023 and is stable.

This MR also removes Notify.provisioned_member_access_granted_email mailer that hasn't been used since Oct 20, 2023 because enterprise_users_automatic_claim FF was globally enabled. It should be safe to remove this mailer.

Related to #421407 (closed) and #424226 (closed)

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