xfrm: Remove duplicate assignment

The statement in the switch is repeated with the statement at the
beginning of the while loop, so this statement is meaningless.

The clang_analyzer complains as follows:

net/xfrm/xfrm_policy.c:3392:2 warning:

Value stored to 'exthdr' is never read
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -3392,7 +3392,6 @@ decode_session6(struct sk_buff *skb, struct flowi *fl, bool reverse)
 		case NEXTHDR_DEST:
 			offset += ipv6_optlen(exthdr);
 			nexthdr = exthdr->nexthdr;
-			exthdr = (struct ipv6_opt_hdr *)(nh + offset);
 			break;
 		case IPPROTO_UDP:
 		case IPPROTO_UDPLITE: