Go to the Playlist, tap on the 3 dots next to the song title you want to remove. 
(in previous iReal Pro versions, tap & hold or swipe the song title)
After that select from these options: 
 - Remove it from the playlist
 - Move to Trash. This removes the song from the playlist as well as removing it from the song library (moved to Trash). It will remove the song from any other playlist it is in.