Get rid of HippoRepositoryServer

HippoRepositoryServer is not used for anything and (probably) never was. 

And keeping it is tricky and confusing since we now also have an enterprise module called hippo-enterprise-repository (with an HippoEnterpriseRepository subclass of LocalHippoRepository). 

The HippoRepositoryServer class (in repository-engine module) can simply be deleted, and everything related to it (test classes, possible test resources).