src/Cart/ShoppingCartController.php
public function removeall($request)
    {
        if ($product = $this->buyableFromRequest()) {
            $this->cart->remove($product, null, $request->getVars());
        }

        $this->updateLocale($request);
        $this->extend('updateRemoveAllResponse', $request, $response, $product);
        return $response ? $response : self::direct();
    }
public function removeall($request)
    {
        if ($product = $this->buyableFromRequest()) {
            $this->cart->remove($product, null, $request->getVars());
        }

        $this->updateLocale($request);
        $this->extend('updateRemoveAllResponse', $request, $response, $product);
        return $response ? $response : self::direct();
    }
Action: remove all of an item from the cart

@param HTTPRequest $request

@return HTTPResponse
@throws \SilverStripe\Control\HTTPResponse_Exception