Cleanup add_refresh_pull_mirror_worker feature flag

What does this MR do and why?

Contributes to #391934 (closed)

The feature flag is globally enabled since 2023-02-16

Problem

After user downgrades their payment plan to Free, then the pull mirroring configuration is disabled. It's not re-enabled after user upgrades back and it requires extra efforts to enable it for each project.

Solution

Automatically re-enable pull mirroring configurations after the subscription upgrade.

MR acceptance checklist

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