Change shift-delete to trigger core:cut in Win32 and Linux keymaps

This behavior is standard WIndows and Linux for shift-delete.

Previously shift-delete was a redundant mapping, anyway. So this brings it further in line with expected behavior for Windows and Linux users.
![image](https://cloud.githubusercontent.com/assets/7287076/3550681/6cae3d78-08dc-11e4-8ea4-61e81a35d5d5.png)