tcp : remove assignment without effect

rstreason is only relevant in the code paths with the label
'dropwithreset', but not in the one with the label 'drop'.
No functional change intended.

Reviewed by:		Nick Banks, rrs, Peter Lei, imp
MFC after:		1 week
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D51814

Author: tuexen

sys/netinet/tcp_input.c

@@ -2218,7 +2218,6 @@ tcp_do_segment(struct tcpcb *tp, struct mbuf *m, struct tcphdr *th,
 		    SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) {
 			tcp_log_end_status(tp, TCP_EI_STATUS_RST_IN_FRONT);
 			tp = tcp_drop(tp, ECONNRESET);
-			rstreason = BANDLIM_UNLIMITED;
 		} else {
 			tcp_ecn_input_syn_sent(tp, thflags, iptos);
 			tcp_send_challenge_ack(tp, th, m);