Simply remove: operation.get("principal").remove(username); to the example above. This just remove a simple parameter.
You maybe want to look into jboss-dmr library for supported operations: https://github.com/jbossas/jboss-dmr/blob/master/src/main/java/org/jboss/dmr/ModelNode.java
See the snapshot of the implementation from above class. Hopes this helps.
running jboss-cli with the --gui parameter is also handy for learning the structure of the commands (navigate to the datasource, right-click for operations, see what is generated)