dbtables.py: use os.unlink instead of temporary_file.unlink
This PR solves the issue seen in kipp.cannon/python-ligo-lw#14.
@kipp.cannon, this is currently a blocker for the python 3 port of gstlal, as the offline analysis makes heavy use of this feature through the --tmp-space
option in various jobs.
Closes kipp.cannon/python-ligo-lw#14.