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 4a8f3aad37)

sys/netinet/tcp_input.c

@@ -2699,7 +2699,6 @@ enter_recovery:
 						 */
 						tp->sackhint.prr_delivered =
 						    tp->sackhint.sacked_bytes;
-						tp->sackhint.sack_bytes_rexmit = 0;
 						tp->sackhint.recover_fs = max(1,
 						    tp->snd_nxt - tp->snd_una);
 					}