Remove the dependency on a Worker from evaluate

Also remove dead code, `rootSimpleEval` is no longer used since the introduction of dual net.
`iterBestValue` is also no longer used in evaluate and can be reduced to a local variable.

closes https://github.com/official-stockfish/Stockfish/pull/4979

No functional change

src/uci.cpp

@@ -270,7 +270,7 @@ void UCI::trace_eval(Position& pos) {
 
     Eval::NNUE::verify(options, evalFiles);
 
-    sync_cout << "\n" << Eval::trace(p, *threads.main_thread()->worker.get()) << sync_endl;
+    sync_cout << "\n" << Eval::trace(p) << sync_endl;
 }
 
 void UCI::search_clear() {