Remove REGISTRY_FF_ENFORCE_LOCKFILES
## Context https://gitlab.com/gitlab-org/container-registry/-/issues/1335+ introduced the feature flag `REGISTRY_FF_ENFORCE_LOCKFILES` which prevents the registry from starting if any of the lock files exists `database-in-use` and `filesystem-in-use` and the wrong configuration is used. ## Implementation Once we are done rolling out the FF https://gitlab.com/gitlab-org/container-registry/-/issues/1438+, we should remove it from the code base.
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