powerpc/64s: Drop no longer used IDLE_STATE_ENTER_SEQ

There are no longer any callers of IDLE_STATE_ENTER_SEQ, all callers
use IDLE_STATE_ENTER_SEQ_NORET. So drop the former.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Split out of larger patch, write change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>