Remove readonly tmpdir hack post python 3.9

Due to a change in behaviour with pytest 7.3, we had to introduce an explicit tmpdir removal in MDAnalysisTests/coordinates/test_xdr.py as part of #4122. This seems to be caused by some interplay between Windows, py3.9 Pathlib, and Pytest ~7.3.

Once we are done with Python 3.9, we should aim to remove this temporary code.