tcp: Remove the documented dependency of TCPHPTS for RACK and BBR

Since TCPHPTS is now included in the GENERIC kernel, remove the
documented dependency of it from the tcp_rack(4) and tcp_bbr(4)
manual pages.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D43084

(cherry picked from commit 4fb5eda6493c3dada695efbfad0a44d204b7fc5e)