pkg-download: get rid of DL_MODE

The DL_MODE variable is now no longer used with any other value than
"DOWNLOAD", so it no longer makes sense to have this variable at
all. Therefore, this commit gets rid of it.

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