Reduce the number of accumulator states

Reduce from 3 to 2. Make the intent of the states clearer.

STC: https://tests.stockfishchess.org/tests/view/60c50111457376eb8bcaad03
LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 61888 W: 5007 L: 4944 D: 51937
Ptnml(0-2): 164, 3947, 22649, 4030, 154

LTC: https://tests.stockfishchess.org/tests/view/60c52b1c457376eb8bcaad2c
LLR: 2.94 (-2.94,2.94) <-2.50,0.50>
Total: 20248 W: 688 L: 618 D: 18942
Ptnml(0-2): 7, 551, 8946, 605, 15

closes https://github.com/official-stockfish/Stockfish/pull/3548

No functional change.

src/nnue/nnue_accumulator.h

@@ -25,14 +25,11 @@
 
 namespace Stockfish::Eval::NNUE {
 
-  // The accumulator of a StateInfo without parent is set to the INIT state
-  enum AccumulatorState { EMPTY, COMPUTED, INIT };
-
   // Class that holds the result of affine transformation of input features
   struct alignas(CacheLineSize) Accumulator {
     std::int16_t accumulation[2][TransformedFeatureDimensions];
     std::int32_t psqtAccumulation[2][PSQTBuckets];
-    AccumulatorState state[2];
+    bool computed[2];
   };
 
 }  // namespace Stockfish::Eval::NNUE