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.

include/nat.h

@@ -120,12 +120,10 @@ struct natstr {
 
 	/* nation relation codes */
 #define AT_WAR		0
-#define SITZKRIEG	1
-#define MOBILIZATION	2
-#define HOSTILE		3
-#define NEUTRAL		4
-#define FRIENDLY	5
-#define ALLIED		6
+#define HOSTILE		1
+#define NEUTRAL		2
+#define FRIENDLY	3
+#define ALLIED		4
 
 	/* nation reject codes */
 #define REJ_TELE	bit(0)	/* dont allow telegrams to be sent */