Useless use of cat
Remove libspotify dependency

The web API can now delete tracks from playlists. This means that
libspotify is only providing one feature that cannot be replaced by the
web API: deleting local tracks (that don't have a Spotify ID) from
playlists.

The usefulness of that feature is outweighed by the inconvenience of
libspotify - it requires a premium account, you have to manage a
separate set of API credentials, and it takes longer to install.

Remove libspotify, replacing the track-deletion features with the
spotipy client.
Make README slightly more informative