TCP RACK: remove redundant check

No functional change intended.

Reviewed by:		rrs
CID:			1523811
MFC after:		1 week
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D48216

sys/netinet/tcp_stacks/rack.c

@@ -22978,7 +22978,7 @@ rack_process_option(struct tcpcb *tp, struct tcp_rack *rack, int sopt_name,
 
 	switch (sopt_name) {
 	case TCP_RACK_SET_RXT_OPTIONS:
-		if ((optval >= 0) && (optval <= 2)) {
+		if (optval <= 2) {
 			rack_init_retransmit_value(rack, optval);
 		} else {
 			/*