Remove BlankNode requirements from RDFTermFactory.createGraph()

RDFTermFactory.createGraph
says:

> BlankNode objects added to the Graph returned from this method SHOULD be mapped using the createBlankNode(String) of this factory, called using the BlankNode.uniqueReference() as the parameter, before they are inserted into the Graph.

I suggest to remove this paragraph as it is very specific, and I disagree on the need for such a mapping. It is also strange for the RDFTermFActory to put requirements on later calls on the methods of Graph. The pattern of calling createBlankNode with a uniqueReference() is not shown elsewhere.

Note that simple does such a mapping, so this bug would also imply to change this:

https://github.com/apache/incubator-commonsrdf/blob/master/simple/src/main/java/org/apache/commons/rdf/simple/GraphImpl.java#L81

.. but I think COMMONSRDF-15 should decide on that matter.