KVM: x86: remove misplaced comment on active_mmu_pages

The 'mmu_page_hash' is used as hash table while 'active_mmu_pages' is a
list. Remove the misplaced comment as it's mostly stating the obvious
anyways.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210226061945.1222-1-dongli.zhang@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>