lastgenre: force real lookup during import only if no custom whitelist set.
Revert "lastgenre: force real lookup during import only if no custom whitelist set."

This reverts commit 4be30fca2e08d63eb4bfc21f7704fb95e5ef4d26.
lastgenre: invert 'force' option default value.
Merge pull request #254 from KraYmer/lastgenre-skip

lastgenre: force real lookup during import only if no custom whitelist set