Remove Quantity from invoice

Look at application/modules/invoices/views/partial_item_table.php

     <?php foreach ($items as $item) { ?>
                ....
                    <td class="td-amount td-quantity">
                        <div class="input-group">
                            <span class="input-group-addon"><?php _trans('quantity'); ?></span>
                            <input type="text" name="item_quantity" class="input-sm form-control amount"
                                   value="<?php echo format_amount($item->item_quantity); ?>"
                                <?php if ($invoice->is_read_only == 1) {
                                    echo 'disabled="disabled"';
                                } ?>>
                        </div>
                    </td>
               ....

    <?php } ?> 

You should also look at the invoice controller and force the quantity to 1 for each item.