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/common/nat.c

@@ -42,8 +42,7 @@
 
 char *relates[] = {
     /* must follow nation relation defines in nat.h */
-    "At War", "Sitzkrieg", "Mobilizing", "Hostile", "Neutral", "Friendly",
-    "Allied"
+    "At War", "Hostile", "Neutral", "Friendly", "Allied"
 };
 
 char *