package: Remove trailing slash from all package site URLs

The recommended form is without the trailing slash, and will become
mandatory in a coming commit.

This avoids the need for the $$($(2)_SITE:/=) magic in package/pkg-generic.mk
to avoid double slashes in download URLs, like
"https://mosh.mit.edu//mosh-1.2.5.tar.gz

".
                     ^^

Note: this work has already been done in b0b96065 a few
months ago and earlier in c7f4b964 and 4a9eb20d,
but no check has been added at that time to avoid new slashes to slip
in, and so they did. This time a patch will follow immediately to
prevent future mistakes from being unnoticed.

Mass-replaced with the following command:

  git grep -l '_SITE.*/$' | xargs sed -i '/_SITE.*=/s|/$||'

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>