This removes the test-download method for all but
promises-aplus. They all only contain legacy code for
deleting old archives.

Only test262 needs to prepare sources on swarming, which is
moved to a new method, called unconditionally.

All references to --download-data and --download-data-only
in the infrastructure can be removed after this.

BUG=

Review-Url: https://codereview.chromium.org/2227613002
Cr-Commit-Position: refs/heads/master@{#38439}