rgw: remove swift user manifest (DLO) hash calculation

Fixes: #9973
Backport: firefly, giant

Previously we were iterating through the parts, creating hash of the
parts etags (as S3 does for multipart uploads). However, swift just
calculates the etag for the empty manifest object.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>