Cleanup code after removal of classical evaluation

This includes the following changes:
- Remove declaration of removed global variable
- Adapt string that mentions removed UCI option

closes https://github.com/official-stockfish/Stockfish/pull/4675

No functional change

src/evaluate.h

@@ -33,7 +33,6 @@ namespace Eval {
   std::string trace(Position& pos);
   Value evaluate(const Position& pos);
 
-  extern bool useNNUE;
   extern std::string currentEvalFileName;
 
   // The default net name MUST follow the format nn-[SHA256 first 12 digits].nnue