gvtest.py: fix: use os.remove instead of pathlib.unlink to remove a file

gvtest.py: fix: use os.remove instead of pathlib.unlink to remove a file

There were two problems with using pathlib.unlink here:

  1. If the caller did not pass in a value for `dst`, it was assigned to the
     second return value of a `tempfile.mkstemp` call. That is, `dst` would be a
     string, not a `pathlib.Path`, and calling `.unlink()` would fail
     cryptically.

  2. The `missing_ok` parameter which this code was using was only added to
     `pathlib.unlink` in Python 3.8. The stated Python baseline for Graphviz
     currently is Python 3.6. If this code was reached with Python < 3.8 it
     would result in an exception, obscuring the original problem.