west: blobs: Default to listing and fetching all blobs

Instead of both listing and fetching only the blobs in the
modules listed by the user in the command-line, default to
listing/fetching them all for ease-of-use and consistency with other
commands. Remove the --all options since it's meaningless now.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>