rgw: data sync accepts ERR_PRECONDITION_FAILED on remove_object()

rgw: data sync accepts ERR_PRECONDITION_FAILED on remove_object()

sync of deletes uses an If-UnModified-Since precondition, but does not
handle the corresponding ERR_PRECONDITION_FAILED error. treating this as
a failure means that we'll keep retrying the delete which will never
succeed. break this loop by treating ERR_PRECONDITION_FAILED as a
success

Fixes: http://tracker.ceph.com/issues/37448
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit f9d0ccf0069cb1a50621bdf8522038ab371f2a51)