pkg-{download, generic}: remove source-check

This feature is not used by anyone in the core developpers and makes a
drastic simplification of the pkg-download infrastructure harder.

The future patch will move much of what's in the current pkg-download.mk
file into the dl-wrapper which is a shell script.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>