download: get rid of trailing slash removal hack for FOO_SITE

Not needed anymore since the URLs have been cleared, the manual states
they are illegal and a check has been introduced to notice future
mistakes.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>