AS 6 will not support FIELD as PojoCache does not work with Infinispan and no Infiinspan based equivalent exists yet. So:

Add comments to the 4.0, 4.2 and 5.0 jboss-web dtds and to the 5.0 schema noting that it's removed in AS 6.
Remove it altogether as a replication-granularity option in the 6.0 schema, plus remove the replication-field-batch-mode element.