You can find this in admin/invoice.php and remove the red parts:

<tr>
<td><table border="0" width="100%" cellspacing="0" cellpadding="2">
<tr class="dataTableHeadingRow">
<td class="dataTableHeadingContent" colspan="2"><?php echo TABLE_HEADING_PRODUCTS; ?></td>
<td class="dataTableHeadingContent"><?php echo TABLE_HEADING_PRODUCTS_MODEL; ?></td>
<td class="dataTableHeadingContent" align="right"><?php echo TABLE_HEADING_TAX; ?></td>
<td class="dataTableHeadingContent" align="right"><?php echo TABLE_HEADING_PRICE_EXCLUDING_TAX; ?></td>
<td class="dataTableHeadingContent" align="right"><?php echo TABLE_HEADING_PRICE_INCLUDING_TAX; ?></td>
<td class="dataTableHeadingContent" align="right"><?php echo TABLE_HEADING_TOTAL_EXCLUDING_TAX; ?></td> <td class="dataTableHeadingContent" align="right"><?php echo TABLE_HEADING_TOTAL_INCLUDING_TAX; ?></td>
</tr>


and here:

echo ' </td>' . "\n" .
' <td class="dataTableContent" valign="top">' . $order->products[$i]['model'] . '</td>' . "\n";
echo ' <td class="dataTableContent" align="right" valign="top">' . zen_display_tax_value($order->products[$i]['tax']) . '%</td>' . "\n" . ' <td class="dataTableContent" align="right" valign="top"><b>' .
$currencies->format($order->products[$i]['final_price'], true, $order->info['currency'], $order->info['currency_value']) .
($order->products[$i]['onetime_charges'] != 0 ? '<br />' .



Remember admin files aren't overridable.