tcp: for LRD move sysctl from tcp.do_lrd tp tcp.sack.lrd, remove sockopt

Moving lrd sysctl to the tcp.sack branch, since LRD only works with SACK.
Remove the sockopt to programmatically control LRD per session.

Reviewed By: transport, tuexen, rrs
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D42851