test: remove unused code from script_tests

This has been unused since #29648.
Noticed while running a newer version of clang-tidy (19.1.1):
```bash
[127/391][6.2s] /opt/homebrew/opt/llvm/bin/clang-tidy -p=build -quiet --config-file=/bitcoin/src/.clang-tidy /bitcoin/src/test/script_tests.cpp
bitcoin/src/test/script_tests.cpp:126:25: error: local copy 'tx2' of the variable 'tx' is never modified and never used; consider removing the statement [performance-unnecessary-copy-initialization,-warnings-as-errors]
  126 |     CMutableTransaction tx2 = tx;
      |     ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
  127 |     BOOST_CHECK_MESSAGE(VerifyScript(scriptSig, scriptPubKey, &scriptWitness, flags, MutableTransactionSignatureChecker(&tx, 0, txCredit.vout[0].nValue, MissingDataBehavior::ASSERT_FAIL), &err) == expect, message);
512 warnings generated.
```

Author: fanquake

src/test/script_tests.cpp

@@ -123,7 +123,6 @@ void DoTest(const CScript& scriptPubKey, const CScript& scriptSig, const CScript
     ScriptError err;
     const CTransaction txCredit{BuildCreditingTransaction(scriptPubKey, nValue)};
     CMutableTransaction tx = BuildSpendingTransaction(scriptSig, scriptWitness, txCredit);
-    CMutableTransaction tx2 = tx;
     BOOST_CHECK_MESSAGE(VerifyScript(scriptSig, scriptPubKey, &scriptWitness, flags, MutableTransactionSignatureChecker(&tx, 0, txCredit.vout[0].nValue, MissingDataBehavior::ASSERT_FAIL), &err) == expect, message);
     BOOST_CHECK_MESSAGE(err == scriptError, FormatScriptError(err) + " where " + FormatScriptError((ScriptError_t)scriptError) + " expected: " + message);