Fix CoapRequestBuilder about BootstrapDeleteRequest request which could target "none root" path

[sbernard31]

From (https://www.eclipse.org/lists/leshan-dev/msg01023.html)
In commit fe58a35 ("BootstrapDelete Request can target "none root" path.") has been added possibility to delete "none-root" objects during bootstrap procedure
As I understand this change must be synchronized with method org.eclipse.leshan.server.californium.impl.CoapRequestBuilder.visit(BootstrapDeleteRequest request)
where COAP request is being formed :

    @Override
    public void visit(BootstrapDeleteRequest request) {
        coapRequest = Request.newDelete();
        coapRequest.setConfirmable(true);
        EndpointContext context = EndpointContextUtil.extractContext(destination);
        coapRequest.setDestinationContext(context);
        // here probably we have to call setTarget(coapRequest, request.getPath()); 
        // by analogue with rest methods of visitor
    }