KVM: Simplify kvm_free_memslot() and all its descendents

Now that all callers of kvm_free_memslot() pass NULL for @dont, remove
the param from the top-level routine and all arch's implementations.

No functional change intended.

Tested-by: Christoffer Dall <christoffer.dall@arm.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>