tcp rack: remove references to rb trees

The references should have been removed in
https://cgit.freebsd.org/src/commit/?id=030434acaf4631c4e205f8bccedcc7f845cbfcbf

Reviewed by: rscheff, zlei
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D42386