Pretty straightforward: if you are viewing a playlist that you have the rights to modify, opening the '3 dots' menu for a song should have a 'Remove from this playlist' option.
This helps with the fact that playlist editing in mobile is clunky right now, and also provides for an uniform experience since the '3 dots' menu has an "Add to playlist" option, thus allowing the inverse operation to be available.
Updated on 2019-09-26
This idea has been submitted one year ago, but unfortunately hasn’t gained enough votes during that time (100 votes/year). In order to keep an overview of the active ideas in this forum, this idea will be closed for now. Note that this does not mean the idea has been declined by Spotify.
Please feel free to re-post this idea if you still feel you want this implemented. With a little change in presentation, maybe this time it will gain the necessary support from the community.
For more info: