Add an unbind method on Graph and Namespace Manager to unbind prefixes

[edmondchuc]

Unbinding prefixes works on a Graph object with the memory store by passing None as the value and setting replace=True. If this is not a side effect and is the intended behaviour of unbinding prefixes, we can create an unbind method with the following implementation on a Graph.

def unbind(self, prefix: str):
    self.namespace_manager.bind(prefix, None, replace=True)

Will need to check whether this is the intended behaviour and whether this works consistently across store implementations.

An example on how to unbind a prefix: #2643 (comment)