pkg-download: remove support for the SHOW_EXTERNAL_DEPS DL_MODE

Now that the external-deps implementation relies on the per-package
<pkg>-all-external-deps and <pkg>-external-deps targets and no longer
on the 'source' target with a custom DL_MODE, we can get rid of the
support for the SHOW_EXTERNAL_DEPS DL_MODE value in the pkg-download
logic.

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