To remove content from Offline mode, simply go to the left menu on your Android or iOS device, select offline content.
Here you can see all of the content available for offline use. Select any item that you do not wish to have saved to your phone's storage. Open the playlist or album, turn the Offline Toggle to OFF. By doing so the content will automatically be removed.
This can be used to free up space on your phone, also a way to remove old playlists or albums and update them with some fresh new music that you can enjoy on the go.