This reverts "s3: fix DisableChecksum condition" which introduced the
problem.
This reverts commit c05bb63f9679e09ecb078e7477f3efd02846c2ad.
The code was correct as it stands - the comment was incorrect and this
commit updates it.
See: https://forum.rclone.org/t/s3-upload-md5-check-sum/13706