fix #11687: removeGroups can take unloaded experimenters

Adjust `removeGroups` to accept unloaded experimenters and adjust integration tests to always give unloaded experimenters. Check that http://hudson.openmicroscopy.org.uk/job/OmeroJava-integration-develop/lastSuccessfulBuild/testngreports/integration/integration.AdminServiceTest/ still passes. Also check for relevant tests in #1771.