test: removes unnecessary check from validation_tests

An unnecessary check was added to the block mutation tests
in #29412 where IsBlockMutated is returning true for the invalid
reasons: we try to check mutation via transaction duplication,
but the merkle root is not updated before the check, therefore
the check fails because the provided root and the computed root
differ, but not because the block contains the same transaction twice.

The check is meaningless so it can be removed.

Author: sr-gi

src/test/validation_tests.cpp

@@ -233,7 +233,6 @@ BOOST_AUTO_TEST_CASE(block_malleation)
         // Block with two transactions is mutated if any node is duplicate.
         {
             block.vtx[1] = block.vtx[0];
-            BOOST_CHECK(is_mutated(block, /*check_witness_root=*/false));
             HashWriter hasher;
             hasher.write(block.vtx[0]->GetHash());
             hasher.write(block.vtx[1]->GetHash());