[WIP/RFC] bpo-15872: tests: remove oddity from test_rmtree_errors (GH-22967)

This was added for (some) Windows buildbots back in 2012, and should
either not be necessary anymore, or it should probably get investigated
why "\*.*" gets added to filenames in the first place.

Ref:

Automerge-Triggered-By: GH:hynek