qdev: Legacy properties are now unused internally, drop

Legacy properties are an accidental and undocumented external
interface.  qom-set doesn't work for them (no .set() method).  qom-get
and qom-list-get work only when the underlying qdev property has a
.print() method, i.e. the PCI address properties, as explained in the
previous commit.  Here's one that works:

    (qemu) qom-get /machine/unattached/device[3]/pm addr
    11
    (qemu) qom-get /machine/unattached/device[3]/pm legacy-addr
    "01.3"

And here's one that doesn't:

    (qemu) qom-get /machine/unattached/device[3]/pm bus
    "/machine/i440fx/pci.0"
    (qemu) qom-get /machine/unattached/device[3]/pm legacy-bus
    Error: Property 'PIIX4_PM.legacy-bus' is not readable

Actual use of this undocumented interface seems quite unlikely.  A
deprecation period seems unnecessary.  Drop it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20251022101420.36059-4-armbru@redhat.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
[Commit message improved]