[autotest] delete stage dir for attempt to download a non-existing build

The leaking staged.timestamp and stage dir is causing script used in lab to
consider a non-existing build to be the latest build. Clean up such file/
directory helps to avoid such confusion.

BUG=chromium:229091
TEST=tested with download url in local devserver:
http://dshi.mtv:8080/download?archive_url=gs://chromeos-image-archive/x86-
generic/R17-1208.0.0-a1-b338
DEPLOY=none

Change-Id: Ifce8a75a1d891471875eabed0f434bd5aaf710e3
Reviewed-on: https://gerrit.chromium.org/gerrit/60165
Tested-by: Dan Shi <[email protected]>
Reviewed-by: Chris Sosa <[email protected]>
Commit-Queue: Dan Shi <[email protected]>
1 file changed