[IPVS]: Remove declaration of unimplemented method and remove unused definition from include/net/ip_vs.h

In include/net/ip_vs.h:
- The ip_vs_secure_tcp_set() method is not implemented anywhere.
- IP_VS_APP_TYPE_FTP is an unused definition.
Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>