Portfolio has no way to know if a keyword was added by a user manually tagging it, or from extraction of what’s embedded in a file, or added from the ‘keywords from path’ feature. They’re all just keywords on a record. Because there’s no way to differentiate between these methods of adding keywords, it won’t remove a keyword that was previously added from the path, as it doesn’t know which they are! There’s also the risk with such a feature that it removes a keyword you may still want on a file.
For that reason removing keywords is a manual task, unless you delete the record and let it recatalog from the new location, creating new keywords based on that new path. On existing records though you can ‘extract properties’ to have it generate the new keywords for the new location without recataloguing, but it won’t automatically clean up the old ones.
Beyond that, you should submit a feature request via our online contact form explaining how such a feature would assist in your workflow, what problem it would help to solve, and any other pertinent information you have about how you use Portfolio.