Just go in your source and search everything that is the spell id of that skill and delete it :D
for GmRobe i dont see why you want to delete that , just go into your backup.sql ( like if you use navicat go in the items table or phpmyadmin in the items table click Search
and look up GmRobe or the item id for it and delete it from the database.. if you do that you will probably get errors when trying to create a GmRobe but it wont create one at all