tcp: remove incorrect reset of SACK variable in PRR
Reviewed By: transport, rrs, tuexen
PR: 253848
MFC after: 3 days
Sponsored By: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29083
(cherry picked from commit 4a8f3aad37dd35c905e34b64b022d60844ba8d01)