kvm: Delete prototype for non-existent function complete_pio

complete_pio ceased to exist in commit
7972995b

 ("KVM: x86 emulator: Move
string pio emulation into emulator.c"), but the prototype remained.
Remove its prototype.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Gleb Natapov <gleb@redhat.com>