OK, so I found where the settings are coming from and can describe how to fix it but I'm not sure you really want to go fiddling there because the settings are very deep in the tree. The shape masters are complicated things and that shape in particular is a nested (group inside a group inside a group) structure where each shape (or group) has its own shapesheet.
I don't know how much you know about Visio so bear with me because I will include some background info in order to describe the editing steps.
We can modify that one shape but the corner will still be there when people add the same shape master again so it is best to modify the master. To do this, go to your Shapes Pane and choose More Shapes > Show Document Stencil. In that stencil, right click on the Manager Shapetacular > Edit Master > Edit Master Shape. This will open a new window showing just that shape master.
Next you need to ensure you can see the Master Explorer. If you don't see that popup, you can turn it on by going to the Developer Tab and checking it there. If you don't see the Developer Tab, you can turn it on in the File > Options > Customize Ribbon.
The Master Explorer shows a tree view and you keep clicking on the + buttons to expose all the children shapes inside this shape. You need to find the very last child shape called Sheet.9 and right click it and select Show Shapesheet.
In the window that appears, scroll to see the section called Geometry 1. Right click on Row 8 and select Delete Row. Do the same to Rows 7 then 6 then 5. Now select the Y cell in Row 4 and delete all that text and change it to "=Height". It should now look like this...
Close the shape sheet and then close the Shape Window. You will be asked if you want to update the master and all instances - choose Yes.