remove PREFERRED capabilities

[mwhudson]

Way back when I added support for core boot classic installs, we did not have a way to indicate whether the installed model preferred to be installed encrypted or unencrypted. Roll time on 9 months or so and our API now supports a list of capabilites and we can define that we can indicate which is preferred by putting it first in the list.

One part of the implementation is a bit delicate really -- relying on list.sort() being stable -- so maybe we should clean that up. But I think the other changes make everything nicer.

[mwhudson]

cc @d-loose for API impact -- we don't have to land this on any particular time frame. but I'm not sure the desktop client will care.