dspace-rdf was developed as standalone module. At the end of the
development most of it was moved to be included into dspace-api. Instead
of using Spring it used its own class that instantiated all necessary
classes. This PR solves the problem described in DS-3190 by refactoring
org.dspace.rdf to adapt patterns used in DSpace 6. It introduces
[dspace]/config/spring/api/rdf.xml to configure spring to instantiate
all necessary classes. org.dspace.rdf.RDFConfiguration was removed
completely, the configuration keys are centralised in
org.dspace.rdf.RDFUtil. Instead of org.dspace.rdf.RDFConfiguration and
DSpace's old ConfigurationManager the new ConfigurationService is now
used everywhere. Configuration properties shouldn't be hold in variables
anymore so that RDF profits from the new autoreload function from
DS-2654.
