The first simple option: Right Click->Convert to Pixels and work as usual (selections and manipulation).
The second option:
Select the shape, evoke right click menu as on the picture, and choose Make Editable:
Next, you delete the part you want (select 1 anchor point and press delete - the anchor point and its surrounding points will be deleted if you press the delete button repeatedly):
UPDATE: how to create the "arc" in question.
This is my way to create your shape, probably there are others.
- Choose pen tool and make a 4-clicks shape as on the picture:
to leave the shape "open" you click escape after 4th click.
- Create 4 additional anchor points by double clicking on the path -
- Choose 2 lower angle anchor points and press Delete -
- Select the shape and Open the Shapes panel -> click on the options button
the button is on the right lower corner.
- Choose Add Shape -
You have this shape available -
Now, work with this shape us usual, but set the fill to NONE: