Swipe the playlist's name.

Tap on Delete and then select:

- Delete Playlist Only (it will delete the playlist but it won't delete all the songs from your main library as you might have some of those songs in other playlists)

- Move All Songs to Trash (It will move every song in the playlist to the Trash)

 

You can retrieve any deleted songs from the Trash smart playlist, (however you cannot retrieve a playlist with its songs.)