Clean up and simplify some nnue code.

Remove some unnecessary code and it's execution during inference. Also the change on line 49 in nnue_architecture.h results in a more efficient SIMD code path through ClippedReLU::propagate().

passed STC:
https://tests.stockfishchess.org/tests/view/6217d3bfda649bba32ef25d5
LLR: 2.94 (-2.94,2.94) <-2.25,0.25>
Total: 12056 W: 3281 L: 3092 D: 5683
Ptnml(0-2): 55, 1213, 3312, 1384, 64

passed STC SMP:
https://tests.stockfishchess.org/tests/view/6217f344da649bba32ef295e
LLR: 2.94 (-2.94,2.94) <-2.25,0.25>
Total: 27376 W: 7295 L: 7137 D: 12944
Ptnml(0-2): 52, 2859, 7715, 3003, 59

closes https://github.com/official-stockfish/Stockfish/pull/3944

No functional change

bench: 6820724

src/nnue/nnue_feature_transformer.h

@@ -123,8 +123,10 @@ namespace Stockfish::Eval::NNUE {
       // We use __m* types as template arguments, which causes GCC to emit warnings
       // about losing some attribute information. This is irrelevant to us as we
       // only take their size, so the following pragma are harmless.
+      #if defined(__GNUC__)
       #pragma GCC diagnostic push
       #pragma GCC diagnostic ignored "-Wignored-attributes"
+      #endif
 
       template <typename SIMDRegisterType,
                 typename LaneType,
@@ -156,9 +158,9 @@ namespace Stockfish::Eval::NNUE {
 
       static constexpr int NumRegs     = BestRegisterCount<vec_t, WeightType, TransformedFeatureDimensions, NumRegistersSIMD>();
       static constexpr int NumPsqtRegs = BestRegisterCount<psqt_vec_t, PSQTWeightType, PSQTBuckets, NumRegistersSIMD>();
-
+      #if defined(__GNUC__)
       #pragma GCC diagnostic pop
-
+      #endif
   #endif