Stop downloading target.common.tests.zip as it's no longer necessary

[rhcu]

https://queue.taskcluster.net/v1//task/LO3C6RBPR_CPxVHoekVhew/artifacts/public%2Fbuild%2Ftarget.common.tests.zip; latest_cov_build.py throws urllib.error.HTTPError: HTTP Error 404: Not Found.

[rhcu]

Seems that we can skip the download of this artifacts since lcov_rewriter.py is already present in mozilla-central.

[marco-c]

Yes, we should be able to stop downloading it now that we are cloning mozilla-central.