Remove duplicated Position object in UCIEngine

Also fixes searchmoves.

Drop the need of a Position object in uci.cpp.

A side note, it is still required for the static functions,
but these should be moved to a different namespace/class
later on, since sf kinda relies on them.

closes https://github.com/official-stockfish/Stockfish/pull/5169

No functional change

src/benchmark.h

@@ -25,9 +25,7 @@
 
 namespace Stockfish {
 
-class Position;
-
-std::vector<std::string> setup_bench(const Position&, std::istream&);
+std::vector<std::string> setup_bench(const std::string&, std::istream&);
 
 }  // namespace Stockfish