playlistcontrols: Fix playlist deletion
CoreModel used to have a property named active_playlist to keep track of the playlist being played. This property has been superseded by the active_core_object property which keeps track of the object being played (playlist, artist, song or album). However, the delete_action from PlaylistCreation was not updated to use active_core_object. This results in a crash when a playlist is deleted.
This issue is fixed by replacing active_playlist with active_core_object.
Fixes: 2aaa6e61