Moss State: `moss state prune` and `moss state remove` should also affect boot entries

[sammypanda]

When doing a prune or remove i.e. from a bootable drive, it is possible to boot into an initramfs emergency mode when going into a state that has been removed.

relevant to #312

[Nomen-Luni]

As an example moss state prune -k 2 still shows 5 entries in the 'spacebar' boot menu on next boot even though only 2 valid ones remain.