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/assa.c

@@ -173,8 +173,7 @@ assa(void)
 		putland(llp->unit.land.lnd_uid, &llp->unit.land);
 	    } else {
 		pr("%s was spotted", prland(&llp->unit.land));
-		if (rel == HOSTILE || rel == AT_WAR || rel == SITZKRIEG ||
-		    rel == MOBILIZATION) {
+		if (rel <= HOSTILE) {
 		    wu(0, def->own, "%s spy shot and killed in %s.\n",
 		       cname(player->cnum), xyas(def->x, def->y,
 						 def->own));