merkle: remove unused mutated arg from BlockWitnessMerkleRoot

The `mutated` parameter is never used at any call site - all callers pass `nullptr`.
The explicit comment in `validation.cpp` explains the reason:
// The malleation check is ignored; as the transaction tree itself
// already does not permit it, it is impossible to trigger in the
// witness tree.

src/consensus/merkle.cpp

@@ -73,7 +73,7 @@ uint256 BlockMerkleRoot(const CBlock& block, bool* mutated)
     return ComputeMerkleRoot(std::move(leaves), mutated);
 }
 
-uint256 BlockWitnessMerkleRoot(const CBlock& block, bool* mutated)
+uint256 BlockWitnessMerkleRoot(const CBlock& block)
 {
     std::vector<uint256> leaves;
     leaves.resize(block.vtx.size());
@@ -81,7 +81,7 @@ uint256 BlockWitnessMerkleRoot(const CBlock& block, bool* mutated)
     for (size_t s = 1; s < block.vtx.size(); s++) {
         leaves[s] = block.vtx[s]->GetWitnessHash().ToUint256();
     }
-    return ComputeMerkleRoot(std::move(leaves), mutated);
+    return ComputeMerkleRoot(std::move(leaves));
 }
 
 /* This implements a constant-space merkle path calculator, limited to 2^32 leaves. */

src/consensus/merkle.h

@@ -20,9 +20,8 @@ uint256 BlockMerkleRoot(const CBlock& block, bool* mutated = nullptr);
 
 /*
  * Compute the Merkle root of the witness transactions in a block.
- * *mutated is set to true if a duplicated subtree was found.
  */
-uint256 BlockWitnessMerkleRoot(const CBlock& block, bool* mutated = nullptr);
+uint256 BlockWitnessMerkleRoot(const CBlock& block);
 
 /**
  * Compute merkle path to the specified transaction

src/test/fuzz/merkle.cpp

@@ -51,7 +51,7 @@ FUZZ_TARGET(merkle)
     }
 
     // Test ComputeMerkleRoot
-    bool mutated = fuzzed_data_provider.ConsumeBool();
+    bool mutated = fuzzed_data_provider.ConsumeBool(); // output param, initial value shouldn't matter
     const uint256 merkle_root = ComputeMerkleRoot(tx_hashes, &mutated);
 
     // Basic sanity checks for ComputeMerkleRoot
@@ -66,7 +66,7 @@ FUZZ_TARGET(merkle)
     }
 
     if (!block->vtx.empty()){
-        const uint256 block_witness_merkle_root = BlockWitnessMerkleRoot(*block, &mutated);
+        const uint256 block_witness_merkle_root = BlockWitnessMerkleRoot(*block);
         if (tx_hashes.size() == 1) {
             assert(block_witness_merkle_root == uint256());
         }

src/validation.cpp

@@ -3996,7 +3996,7 @@ static bool CheckWitnessMalleation(const CBlock& block, bool expect_witness_comm
             // The malleation check is ignored; as the transaction tree itself
             // already does not permit it, it is impossible to trigger in the
             // witness tree.
-            uint256 hash_witness = BlockWitnessMerkleRoot(block, /*mutated=*/nullptr);
+            uint256 hash_witness = BlockWitnessMerkleRoot(block);
 
             CHash256().Write(hash_witness).Write(witness_stack[0]).Finalize(hash_witness);
             if (memcmp(hash_witness.begin(), &block.vtx[0]->vout[commitpos].scriptPubKey[6], 32)) {
@@ -4109,7 +4109,7 @@ std::vector<unsigned char> ChainstateManager::GenerateCoinbaseCommitment(CBlock&
     int commitpos = GetWitnessCommitmentIndex(block);
     std::vector<unsigned char> ret(32, 0x00);
     if (commitpos == NO_WITNESS_COMMITMENT) {
-        uint256 witnessroot = BlockWitnessMerkleRoot(block, nullptr);
+        uint256 witnessroot = BlockWitnessMerkleRoot(block);
         CHash256().Write(witnessroot).Write(ret).Finalize(witnessroot);
         CTxOut out;
         out.nValue = 0;