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/global/symbol.c

@@ -139,8 +139,6 @@ struct symbol nation_rejects[] = {
 struct symbol nation_relations[] = {
     {-1, "unknown"},
     {AT_WAR, "at-war"},
-    {SITZKRIEG, "sitzkrieg"},
-    {MOBILIZATION, "mobilization"},
     {HOSTILE, "hostile"},
     {NEUTRAL, "neutral"},
     {FRIENDLY, "friendly"},