Remove outdated note in docs

@jprovaznik Should these caveats be removed? New uploads and artifacts go to object storage if it's configured, correct? And if we remove that part, I think we may as well remove the "must be executed manually" part since it doesn't add much value in the context of the doc.

• https://gitlab.com/gitlab-org/gitlab-ce/blob/34694c3afc8e3476f5f070fe1c97aef9119a921b/doc/administration/uploads.md#L203-207

      Currently this has to be executed manually and it will allow you to
      migrate the existing uploads to the object storage, but all new
      uploads will still be stored on the local disk. In the future
      you will be given an option to define a default storage for all
      new files.

• https://gitlab.com/gitlab-org/gitlab-ce/blob/34694c3afc8e3476f5f070fe1c97aef9119a921b/doc/administration/job_artifacts.md#L170-174

      Currently this has to be executed manually and it will allow you to
      migrate the existing artifacts to the object storage, but all new
      artifacts will still be stored on the local disk. In the future
      you will be given an option to define a default storage artifacts for all
      new files.