auth: Remove .get_challenge (only used for security=server)

auth: Remove .get_challenge (only used for security=server)

With NTLMSSP, for NTLM2 we need to be able to set the effective challenge,
so if we ever did use a module that needed this functionlity, we would
downgrade to just NTLM.

Now that security=server has been removed, we have no such module.

This will make it easier to make the auth subsystem async, as we will
not need to consider making .get_challenge async.

Andrew Bartlett