tcp: Use sbuf_drain unconditionally

After making sbuf_drain safe for external use,
there is no need to protect the call.

MFC after: 2 weeks
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29545

(cherry picked from commit 38ea2bd0698e434ec531b97fa94b24db24e16629)
(cherry picked from commit a04906f0273fa6d80eb3ebf22b9b84e53e6b21e0)