KVM: ppc: Remove unmatched kunmap() call

We're not calling kmap() now, so we shouldn't call kunmap() either. This has no
practical effect in the non-highmem case, which is why it hasn't caused more
obvious problems.

Pointed out by Anthony Liguori.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>