powerpc/kvm: Remove comment related to moving PMU code to perf subsystem

Commit aabcaf6a ("KVM: PPC: Book3S HV P9: Move host OS save/restore
functions to  built-in") added a comment in switch_pmu_to_guest
function, indicating possibility of moving PMU handling code
to perf subsystem. But perf subsystem code compilation depends upon
the enablement of CONFIG_PERF_EVENTS whereas, kvm code don't have
any dependency on this config.
Patch remove this comment as switch_pmu_to_guest functionality is
needed even if perf subsystem is disabled.

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220711034927.213192-2-kjain@linux.ibm.com