tcp: remove a superfluous local var in tcp_sack_partialack()
No functional change.
Reviewed By: transport, tuexen
MFC after: 3 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29088
(cherry picked from commit 9a13d9dceee27fc1bf6fb7e58e83c88707665859)