Remove "Unknown" metadata when Retrieve Metadata is disabled

[DenyAll]

Currently, genre, director and producer metadata information is set to "Unknown" even where "Retreive metadata" is disabled for that shared folder.

Suggest that this shouldn't be the case - if metadata retrieval is turned off for a Shared folder then the file shouldn't be included in any categories other than the Folder category.

This is distinct from the case where metadata is enable, but the information is missing - then it makes sense to categorise it as Unknown.