(Cleanup Legacy Storage): remove feature flag for: "Move all projects to hashed storage after rename"

Once gitlab.com is fully on hashed storage, we can remove the feature flag skip_hashed_storage_upgrade and this line https://gitlab.com/gitlab-org/gitlab/blob/master/app%2Fservices%2Fprojects%2Fafter_rename_service.rb#L114

so that all projects get migrated to hashed storage automatically