bootctl: Remove dependency on machine-id.

The machine-id is used to create a few directories and setup a default
loader entry in loader.conf. Having bootctl create the directories
itself is not particularly useful as it does not put anything in them
and bootctl install is not guaranteed to be called before an initramfs
tool like kernel-install so other programs will always need to have
logic to create the directories themselves if they happen to be called
before bootctl install is called.

On top of this, when using unified kernel images, these are installed to
$BOOT/EFI/Linux which removes the need to have the directories created
by bootctl at all. This further indicates that these directories should
be created by the program that puts something in them rather than by
bootctl.

Removing the machine-id dependency allows bootctl install to be called
even when there's no machine-id in the image. This is useful for image
builders such as mkosi which don't have a machine-id when
installing systemd-boot (via bootctl) because it should only be
generated by systemd when the final image is booted.

The default entry in loader.conf based on the machine-id in loader.conf
is also removed which shouldn't be a massive loss in usability overall.

This commit reverts commit 341890de.