refactor: SpanReader without nVersion

The field is unused, so remove it.

This is also required for future commits.

src/wallet/test/wallet_tests.cpp

@@ -759,7 +759,7 @@ BOOST_FIXTURE_TEST_CASE(wallet_descriptor_test, BasicTestingSetup)
     vw << int32_t{0};
     vw << int32_t{1};
 
-    SpanReader vr{0, malformed_record};
+    SpanReader vr{malformed_record};
     WalletDescriptor w_desc;
     BOOST_CHECK_EXCEPTION(vr >> w_desc, std::ios_base::failure, malformed_descriptor);
 }