Cleanup add_refresh_pull_mirror_worker feature flag

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/391934

**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.

Changelog: added