This line is coming from magento. You have to create the local copy of core file and remove it.
1. Make a copy of file
From: app/code/core/Mage/Sales/Block/Order/Totals.php
To: app/code/local/Mage/Sales/Block/Order/Totals.php
after that, remove this part of code
/** * Base grandtotal */
if ($this->getOrder()->isCurrencyDifferent()) {
$this->_totals['base_grandtotal'] = new Varien_Object(array(
'code' => 'base_grandtotal',
'value' => $this->getOrder()->formatBasePrice($source->getBaseGrandTotal()),
'label' => $this->__('Grand Total to be Charged'),
'is_formated' => true,
));
}