Revert Merge pull request #905

[ashleysommer]

Causes exception:

  File "/home/travis/build/RDFLib/rdflib/rdflib/graph.py", line 907, in bind
    return self.namespace_manager.bind(
  File "/home/travis/build/RDFLib/rdflib/rdflib/graph.py", line 326, in _get_namespace_manager
    self.__namespace_manager = NamespaceManager(self)
  File "/home/travis/build/RDFLib/rdflib/rdflib/namespace.py", line 357, in __init__
    self.bind("xml", "http://www.w3.org/XML/1998/namespace")
  File "/home/travis/build/RDFLib/rdflib/rdflib/namespace.py", line 473, in bind
    self.store.bind(prefix, namespace)
  File "/home/travis/build/RDFLib/rdflib/rdflib/plugins/memory.py", line 246, in bind
    self.__namespace.delete(bound_prefix)
AttributeError: 'dict' object has no attribute 'delete'

[gromgull]

re: @akimul