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