Don't use "OwnBook" by default

Stick to UCI protocol that says:

* by default all the opening book handling is done by the GUI,
  but there is an option for the engine to use its own book
  ("OwnBook" option, see below)

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/benchmark.cpp

@@ -72,7 +72,6 @@ void benchmark(int argc, char* argv[]) {
 
   Options["Hash"]    = ttSize;
   Options["Threads"] = threads;
-  Options["OwnBook"] = false;
 
   if (valType == "time")
       limits.maxTime = 1000 * atoi(valStr.c_str()); // maxTime is in ms