You can't delete curves in a sketch that is used as a string in one or more features, e.g. Extrude.
Create the extra needed curves in the sketch, choose add/remove objects from string, using shift-MouseButton 1 to remove objects and MB 1 to add objects.
Make sure your string as closed if it was closed before the edit, then choose "update model" or exit sketch which will also update the model.
After the update, you can remove unused objects from within the sketch.
Erland R. Nielsen
Denmark