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/engine.h

@@ -79,6 +79,9 @@ class Engine {
 
     void        trace_eval() const;
     OptionsMap& get_options();
+    std::string fen() const;
+    void        flip();
+    std::string visualize() const;
 
    private:
     const std::string binaryDirectory;