If you used the Rectangle tool to draw a shape layer rectangle you ended up with a parametric shape. What you have to do is right-click on Rectangle 1 path in the shape layer and select Convert to Bezier. When you have done that you have a path. Under rectangle, you will now have Path 1. Select Rectangle 1 > Path 1, then select the pen tool (g) and hold down the CtrlCmnd key and draw a selection around the two points you want to delete. You'll see the cursor change to an open white arrowhead. When you have two vertices selected right click close to the path and you should have a popup menu that has Mask at the top and then Mask and Shape Path on the second line. Click that and uncheck Closed.
That's it. A little more convoluted than AE but it works just fine.