Simplify start_searching() signature

Retire the "sync" behaviour that now is up to
the caller to honour.

No functional change.

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

src/benchmark.cpp

@@ -119,6 +119,7 @@ void benchmark(int argc, char* argv[]) {
       else
       {
           Threads.start_searching(pos, limits);
+          Threads.wait_for_search_finished();
           nodes += Search::RootPosition.nodes_searched();
       }
   }