Remove missing_ok arg from call to Path.unlink()
Remove missing_ok arg from call to Path.unlink()

Meant to do this in the previous commit. The missing_ok arg isn't
supported in Python 3.6 or 3.7.

Amends d93136c1f2bfa83b24722182d09c097e201115e8

wylee/runcommands,wylee/runcommands