<?xml version="1.0"?>
    <config>
        <modules>
            <Ocra_CartShipping>
                <version>1.3.0</version>
            </Ocra_CartShipping>
        </modules>
        <global>
        <models>
            <ocra_cartshipping>
                <class>Ocra_CartShipping_Model</class>
            </ocra_cartshipping>
        </models>
            <events>
            <sales_quote_collect_totals_before>
                <observers>
                    <ocra_cartshipping>
                        <class>ocra_cartshipping/observer</class>
                        <method>handleShippingMethod</method>
                    </ocra_cartshipping>
                </observers>
            </sales_quote_collect_totals_before>
            </events>
        </global>
    </config>

<?php
class Ocra_CartShipping_Model_Observer {
    public function handleShippingMethod($observer) {


        $quote = $observer->getEvent()->getQuote();
        $shippingAddress = $quote->getShippingAddress();
        $billingAddress = $quote->getBillingAddress();


        if(Mage::app()->getRequest()->getControllerName() == 'checkout_cart'){
            $shippingAddress->setShippingMethod(null);
            $shippingAddress->save();

            $saveQuote = true;
        }
    }
}