Common names cache dir dead code to remove

[thompsonmj]

The common names code has a dead cache dir block that should be removed:

• stale args.cache_dir handling from the standalone resolve_common_names.main() parser path.

The standalone parser does not define --cache-dir, so that block can't provide working standalone cache-dir support as written. The normal packaged CLI path already supports a custom cache dir through the top-level global option:

taxonopy --cache-dir /custom/cache common-names ...