test/boot-image: don't specify boot mode

We might specify it in the future, knowing which mode each image
supports, but for now let's leave it off.

This reverts commit 15e9d28.

test/scripts/boot-image

@@ -86,7 +86,6 @@ def boot_ami(distro, arch, image_type, image_path):
                    "--username", "osbuild",
                    "--ssh-privkey", privkey,
                    "--ssh-pubkey", pubkey,
-                   "--boot-mode" , "uefi",
                    raw_image_path, "test/scripts/base-host-check.sh"]
             runcmd(cmd)