Revert "KVM: MMU: split kvm_mmu_free_page"

This reverts commit bd4c86ea.

There is not user for kvm_mmu_isolate_page() any more.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>