coreos-download: use `parent-pkgdiff` directly from metadata
Previously we need to calculate the pkg diff between releases manually
from the existing build metadata. However, this will not be the case after
the build metadata includes parent-pkgdiff field which already calculates
the pkgdiffs between releases using coreos-assembler. In the future, we
should be able to completely remove the manual calculation logic from frontend
after a few releases and revert back to the same logic as fedora-coreos-browser.
Signed-off-by: Allen Bai <firstname.lastname@example.org>