pkg-download: do not test SITE_METHOD

SITE_METHOD is always set in inner-generic-package. So, it is useless
to test it in pkg-download.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>