[7.2] Remove clustering related things from jboss-ejb3.xml examples

In EAP 7.1, EJBs are by default marked as clustered. Parts of deployment descriptors which mark EJBs as clustered are now ignored (so is the @Clustered annotation, see https://issues.jboss.org/browse/WFLY-2363 for details). We should remove all references to clustering from A.4. jboss-ejb3.xml Deployment Descriptor Reference in the EJB development guide.

That means, please remove

• the section The clustering namespace: urn:clustering:1.0
• the <c:clustering> element at the end of the complete jboss-ejb3.xml example at the end of appendix 4 (that means the <assembly-descriptor> will be empty, so there's no reason to keep it either)