doc: Remove TODO from block fuzz test

src/test/fuzz/block.cpp

@@ -58,8 +58,6 @@ FUZZ_TARGET_INIT(block, initialize_block)
     (void)block.ToString();
     (void)BlockMerkleRoot(block);
     if (!block.vtx.empty()) {
-        // TODO: Avoid array index out of bounds error in BlockWitnessMerkleRoot
-        //       when block.vtx.empty().
         (void)BlockWitnessMerkleRoot(block);
     }
     (void)GetBlockWeight(block);