remove unused methods from duckdb vtab

[myrrc]

1. Remove unused and not set methods from duckdb vtab.
2. Correctly set MAX_THREADS for table function (was u64::MAX but duckdb uses
   another constant).
3. Move boolean definitions like filter pushdown to C++ part.
4. Remove named parameters since we don't support them.
5. Fix a memory leak when throwing errors.