There are several ways in which you can remove the skin:
1. It is better to remove the skin installed from the repository from the software manager, or by using the command
opkg remove enigma2-plugin-skins-x-skin_name
2. Alternatively, you can remove the skin itself physically using a command
rm -R /usr/share/enigma2/skin_name
3. You can also delete the folder with the skin using any convenient ftp client.