tcp: remove debug output from RACK

Reported by: iron.udjin@gmail.com, Marek Zarychta
Reviewed by: rrs
PR: 256538
Differential Revision: https://reviews.freebsd.org/D30723
Sponsored by: Netflix, Inc.

(cherry picked from commit f1536bb53898b12e2d19938f8fe2d04b5e5d12a6)