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/options.c

@@ -59,7 +59,6 @@ int opt_NO_PLAGUE = 1;
 int opt_RAILWAYS = 1;
 int opt_RES_POP = 0;
 int opt_SAIL = 1;
-int opt_SLOW_WAR = 0;
 int opt_SUPER_BARS = 0;
 int opt_TECH_POP = 0;
 int opt_TRADESHIPS = 0;