Remove RADIX_MPATH headers, they were unused since r293159.

MFC after:	2 weeks

sys/netinet6/nd6_nbr.c

@@ -37,7 +37,6 @@ __FBSDID("$FreeBSD$");
 #include "opt_inet.h"
 #include "opt_inet6.h"
 #include "opt_ipsec.h"
-#include "opt_mpath.h"
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -63,9 +62,6 @@ __FBSDID("$FreeBSD$");
 #include <net/if_dl.h>
 #include <net/if_var.h>
 #include <net/route.h>
-#ifdef RADIX_MPATH
-#include <net/radix_mpath.h>
-#endif
 #include <net/vnet.h>
 
 #include <netinet/in.h>