Remove ETAG recommendation from AtomPub section

CMIS 1.0 recommends the support of ETAGs (section "3.2.1 Entity Tag"). The implementation in the context of CMIS is rather difficult and in some cases impossible. Since ETAGs are not supported by many CMIS repositories today and almost no CMIS client takes advantage of them, I would like to remove them from CMIS 1.1.