KVM: mmu: unify destroy_kvm_mmu with kvm_mmu_unload

They do the same thing, and destroy_kvm_mmu can be confused with
kvm_mmu_destroy.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>