Extract EE specific files/lines for spec/migrations

We have the following files containing EE specific code. We should move them to ee/

• spec/migrations/active_record/schema_spec.rb => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19563 / https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6046
• spec/migrations/remove_soft_removed_objects_spec.rb => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19966