Remove option SLOW_WAR

SLOW_WAR has issues:

* The check whether the attacker old-owns the attacked sector is
  broken, because att_abort() uses sect.sct_oldown uninitialized.

  Spotted by the Clang Static Analyzer.

* Its implementation in setrel() is somewhat scary.  It's actually
  okay, because that part of setrel() only runs within decl().  Other
  callers don't reach it: update_main() because player->god != 0
  there, and the rest because they never pass a rel < HOSTILE.

* Documentation is a bit vague.

SLOW_WAR hasn't been used in a public game in years.  Fixing it is not
worth it, so remove it instead.

src/lib/commands/vers.c

@@ -101,8 +101,6 @@ vers(void)
     if (rollover_avail_max)
 	pr("Up to %d avail can roll over an update.\n",
 	   rollover_avail_max);
-    if (opt_SLOW_WAR)
-	pr("Declaring war will cost you $%i\n\n", War_Cost);
     pr("Happiness p.e. requires 1 happy stroller per %d civ.\n",
        (int)hap_cons / etu_per_update);
     pr("Education p.e. requires 1 class of graduates per %d civ.\n",