spapr: Remove obsolete entry_point field from sPAPRMachineState

The sPAPRMachineState structure includes an entry_point field containing
the initial PC value for starting the machine, even though this always has
the value 0x100.

I think this is a hangover from very early versions which bypassed the
firmware when using -kernel.  In any case it has no function now, so remove
it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
