pkg-generic: don't use DL_MODE in .stamp_downloaded

The .stamp_downloaded target is now only being used to really
download, and no longer for other activities like "source check" or
"external deps". So the check on DL_MODE being equal to DOWNLOAD is no
longer necessary.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>