legal-info: remove FOO_MANIFEST_TARBALL and FOO_MANIFEST_SITE defaults

When FOO_SOURCE is non-empty, FOO_MANIFEST_TARBALL is always set.
When FOO_SOURCE is empty, FOO_MANIFEST_TARBALL is not set, but also
never used, due to the if below which defuses the whole legal-info
processing for packages that have FOO_SOURCE explicitly set to an
empty string.

So get rid of the default assignment to "not saved".

Do it for FOO_MANIFEST_SITE as well: it is pointless to have
FOO_MANIFEST_SITE with an empty FOO_SOURCE in a package. A quick
grep session in the sources confirmed this assumption is indeed
true for the current code.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>