The only way you can delete a song from your playlist, is that you first have to find your song on the list (maybe by filtering by name), then next to the name tap the 3 dots and there is the option. That or even a worse way, clicking edit on the playlist and then manually scrolling until you find your song and delete it.
Why? Please tell me why?
Can't you put the option on the 3 dots that appear on the song you are listening right now?
Also, on watchOS, there is a way to unlike songs, but no way to delete from playlist, which seems silly to me since this is a playlist centred music streaming service.
Believe it or not, I'm not using Spotify solely for this reason, it's really cumbersome to manage my library this way.