Fix dangling `tmpdir` reference from cache feature

The added cache feature tried to reference a temporary directory that no
longer existed; this corrects that bug by excising the additional code.

Since this was unused, no ill effects flow from this.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>
Merge pull request #120 from daniel-pittman/bug/master/image-download-tmpdir-error-after-cache-update

Fix dangling `tmpdir` reference from cache feature