Magento – Magento 2 : Getting rid of meta description tag on product pages

You can rewrite the Mage_Catalog_Block_Product_View block and change the method _prepareLayout. That is where the meta description is set.

You need to change this:

        $description = $product->getMetaDescription();
        if ($description) {
            $headBlock->setDescription( ($description) );
        } else {
            $headBlock->setDescription(Mage::helper('core/string')->substr($product->getDescription(), 0, 255));
        }

Into this

$description = $product->getMetaDescription();
$headBlock->setDescription( ($description) );

I see 3 ways to tackle this.
Option 1.
Have a cron that runs every night that builds this meta description for each product. This way you don't interfere with the Magento flow for frontend.

The code can look something like this (untested)

$storeViews = Mage::getModel('core/store')->getCollection();
foreach ($storeViews as $store) {
    if ($store->getId() == 0) {
        continue; //skip admin store
    }
    $collection = Mage::getModel('catalog/product')->setStoreId($store->getId())->getCollection()->addAttributeToSelect('*');
    foreach ($collection as $product) {
        $metaDescription = 'Buy '.$product->getName().' online and save '.CUSTOM LOGIC HERE FOR THE PERCENTAGE.'% today. Free shipping and secure payments';
        Mage::getSingleton('catalog/product_action')->updateAttributes(
             array($product->getId()),
             array('meta_description' => $metaDescription),
             $store->getId()
        )
    }
}

This makes sense if your meta descriptions are going to change a lot and if you don't have a lot of products.

Option 2.
Rewrite the product model Mage_Catalog_Model_Product and add a method called getMetaDescription() like this:

public function getMetaDescription() {
    $name = $this->getName();
    $percentage = ...custom logic for percentage
    return Mage::helper('catalog')->__('Buy %s online and save %s % today. Free shipping and secure payments', $name, $percentage);
}

This will generate the meta description on the fly. It makes sense if you don't have a very complicated discount formula. I would choose this one. I wrote it as the second one (in the middle) to see if you are paying attention. :)

Option 3.
Is very similar to the one above, but involves modifying the block that adds the meta description. You should use this in case you don't want to touch the product model.
You have to rewrite the Mage_Catalog_Block_Product_View and in the method _prepareLayout replace $description = $product->getMetaDescription(); with

$name = $product->getName();
$percentage = ...custom logic for percentage
$description = Mage::helper('catalog')->__('Buy %s online and save %s % today. Free shipping and secure payments', $name, $percentage);