Cleanup Evalfile handling

This cleans up the EvalFile handling after the merge of #4915,
which has become a bit confusing on what it is actually doing.

closes https://github.com/official-stockfish/Stockfish/pull/4971

No functional change

src/nnue/nnue_architecture.h

@@ -37,7 +37,7 @@ namespace Stockfish::Eval::NNUE {
 // Input features used in evaluation function
 using FeatureSet = Features::HalfKAv2_hm;
 
-enum NetSize {
+enum NetSize : int {
     Big,
     Small
 };