You have two options to delete a slide:remove it from the playlist or delete it permanently from your account.
If you just want to remove a slide from a playlist (without deleting it entirely), you can do so directly from the playlist preview side panel:
Open the playlist for editing.
In the left column, below Playlist Preview, individual the slide.
Click on the icon basket next to the slide (see image).
The slide will only be removed from that playlist, but will remain saved in your library.
To completely delete a slide:
Go to your slide library (e.g. in the playlist creation/edit section).
Find the slide you want to delete.
Click on the three dots (•••) above the slide.
Select Remove the menu.