Using `winreg.DeleteTree` in `test_winreg`

[aisk]

There is a recursive function delete_tree in test_winreg to delete the entire key. We can use the newly added winreg.DeleteTree to do this for more robust code and to simplify our code base.

Linked PRs

• gh-142103: Using winreg.DeleteTree to replace delete_tree in test_winreg #142104