KVM: PPC: Book3s: Remove kvmppc_read_inst() function

In the context of replacing kvmppc_ld() function calls with a version of
kvmppc_get_last_inst() which allow to fail, Alex Graf suggested this:

"If we get EMULATE_AGAIN, we just have to make sure we go back into the guest.
No need to inject an ISI into  the guest - it'll do that all by itself.
With an error returning kvmppc_get_last_inst we can just use completely
get rid of kvmppc_read_inst() and only use kvmppc_get_last_inst() instead."

As a intermediate step get rid of kvmppc_read_inst() and only use kvmppc_ld()
instead.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>