tcp cc: remove non-working sctp support

As suggested by lstewart, remove the non-working SCTP support in the
TCP congestion control modules. SCTP has a similar functionality
(although not using kernel loadable modules), on which the TCP stuff
was built on, but the integration was never done.
No functional change intended.

Reviewed by: Peter Lei, cc
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46142