RSVP_ISI

[Maxime Villard <max%m00nbsd.net@localhost>]

There is a lot of dead code under #ifdef RSVP_ISI. I would like to remove it
entirely. My remarks are:

 * RSVP_ISI was added 22 years ago. The commit message says that it is a
   "working snapshot of work in progress". Since, it has almost not moved.

 * If you enabled RSVP_ISI in a config file, the option wouldn't build
   correctly, because there is no defflag.

 * RSVP_ISI makes the code in ip_mroute.c more complicated to understand. In
   fact, it pollutes the whole thing.

 * The functions are not called. Eg ip_rsvp_vif_done() is never called.

 * FreeBSD and OpenBSD both removed it, as general cleanup. The former 8
   years ago, the latter 4 years ago.

Maxime