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
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D42386

(cherry picked from commit b10ae5a9b21aa11e96a1951ccc4525dd1fdb60b3)