arm64: KVM: remove misleading comment on pmu status

Comment about how PMU access is handled is not relavant since v4.6
where proper PMU support was added in.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>