Delete zip files after unpacking

Precinct-level results from the Clarity system all have the same
filename, so we need to delete the file after extracting it so
the file will be re-fetched for the next county.

Addresses https://github.com/openelections/core/issues/124
Don't make url_paths entry for county with no precinct results

Addresses https://github.com/openelections/core/issues/124