Don't release the HNB context as there's plenty of code that
assumes there's always a HNB context associated with a SCTP connection.
Instead, simply unset the hnb_registered flag in the context when
processing a HNB_DE-REGISTER.
Related: OS#5676
Change-Id: Id5c4f5c900ea049f54afbf58edb84b4dc00b1dcb