Magento – How to get cart item id using product id in magento2

This should be done with the Mage_Sales_Model_Quote_Item class. If you're updating several items it might be better to use it's collection tho.

$the_item_ids = array(2,5,6,8);

$collection = Mage::getModel('sales/quote_item')->getCollection()
   ->addFieldToFilter('item_id', array('in' => $the_item_ids));

foreach($collection as $item)
{
  var_dump($item->getData());
}

I added the following custom code in grouped.phtml of path: app\design\frontend[your_theme]\default\template\catalog\product\view\type\grouped.phtml

Below qty code:

    <a href="javascript:void(0)" title="<?php echo Mage::helper('core')->quoteEscape($this->__('Remove This Item')) ?>"
          data-confirm="<?php echo $this->__('Are you sure you would like to remove this item from the shopping cart?') ?>"
           class="remove">
       <?php echo $this->__('Remove Item') ?>
    </a>
    <!-------End of custom code------->

and in same file added the following script to remove product:

jQuery(document).ready(function(){

jQuery('.remove').click(function(){
    jQuery(this).prevAll('input').val(0);
    jQuery(this).closest('tr').hide();
    //alert(jQuery(this).closest('tr').find('td').html());

})

})

This code remove the product from product page as well as from checkout, from grouped product list.