Remove route chaching support from ipsec code. It isn't used for some time.
* remove sa_route_union declaration and route_cache member from struct secashead;
* remove key_sa_routechange() call from ICMP and ICMPv6 code;
* simplify ip_ipsec_mtu();
* remove #include <net/route.h>;

Sponsored by:	Yandex LLC


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275392 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f