This is trivial in Illustrator but I can't figure out if it's possible in Sketch app. In Illustrator you would just select part of a path between 2 anchors and delete it.
select the shape you want to edit. Go to Layer->Paths->Scissors. Click on the path you want to delete. This deletes path between two anchor points. You can add anchor points by double-clicking on the object.
I don't recommend this, but if you're still looking to keep the shape as is instead of using the Scissors tool you can add a Shadow to it.
I also recommend open path option if the shape your deleting is a oval Layer->Path->Open path
