support/download: keep files downloaded without hash

support/download: keep files downloaded without hash

In the situation where the hash is missing from the hash file, the
dl-wrapper downloads the file again and again until the developer
specifies the hash to complete the download step.

To avoid this situation, the freshly-downloaded file is not removed
anymore after a successful download.

After this change, the behaviour is as follows:

- Hash file doesn't exist, or file is in BR_NO_CHECK_HASH_FOR
  => always succeeds.

- Hash file exists, but file is not present
  => file is NOT removed, build is terminated immediately (i.e.
     secondary site is not tried).

- Hash file exists, file is present, but hash mismatch
  => file is removed, secondary site is tried.
  => If all primary/secondary site downloads or hash checks fail, the
     build is terminated.

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
[Arnout: extend commit log]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>