sctp: cleanup handling of graceful shutdown of the peer

Don't handle a graceful shutdown of the peer as an implicit signal
that all partial messages are complete. First, this is not implemented
correctly and second this should not be done by the peer. It is more
appropriate to handle this as a protocol violation.
Remove the incorrect code and leave detecting the protocol violation
and its handling in a followup commit.

(cherry picked from commit 4f14d4b6b7f0ca49b14379e48117121af3ed2669)