Clean-up drop/clear graph functionality of SparqlService

[psiotwo]

Align on the drop graph and clear graph functionality of SparqlService.

---

I guess we need a bit more javadoc. and decide which one we want to do?
note RdfStoreService has a default which is

default void deleteGraph(String graphUri) { //we can overwrite this in the implementation if we have a better way of checking this
executeUpdateQuery(String.format("CLEAR GRAPH <%s>;", graphUri));
}