remove simple eval

No longer needed after the removal of smallnet.

Also remove declaration of `use_smallnet()`.

closes https://github.com/official-stockfish/Stockfish/pull/6799

No functional change

src/evaluate.h

@@ -43,8 +43,6 @@ class AccumulatorStack;
 
 std::string trace(Position& pos, const Eval::NNUE::Network& network);
 
-int   simple_eval(const Position& pos);
-bool  use_smallnet(const Position& pos);
 Value evaluate(const NNUE::Network&           network,
                const Position&                pos,
                Eval::NNUE::AccumulatorStack&  accumulators,