Question: Remove the custom design tab from magento category edit page.![enter image description here](https://i.stack.imgur.com/JAXcy.png)

Answer: 
First of all i am asuming you know how to create module [If not please check [This](https://stackoverflow.com/questions/576908/how-to-create-a-simple-hello-world-module-in-magento) ]

You can remove a tab using the method `Mage_Adminhtml_Block_Widget_Tabs::removeTab()`. Every edit page in Magento (admin side) is actually child block of `Mage_Adminhtml_Block_Widget_Tabs` block class.

To remove tab you have to create extension for that and obsever 

```
adminhtml_catalog_category_tabs

```

As a start up 

```
File : app\code\community\Keyur\RemoveCategoryTab\etc\config.xml

<config>
    <modules>
        <Keyur_RemoveCategoryTab>
            <version>1.0.0</version>
        </Keyur_RemoveCategoryTab>
    </modules>
    <global>
        <models>
            <keyur_RemoveCategoryTab>
                <class>keyur_RemoveCategoryTab_Model</class>
            </keyur_RemoveCategoryTab>
        </models>
    </global>
    <adminhtml>
        <events>
            <adminhtml_catalog_category_tabs>
                <observers>
                    <remove_category_product_tab_from_edit_page>
                        <class>keyur_RemoveCategoryTab/observer</class>
                        <method>removeUnwantedCategoryTabs</method>
                    </remove_category_product_tab_from_edit_page>
                </observers>
            </adminhtml_catalog_category_tabs>
        </events>
   </adminhtml>
</config>

```

Now create observer file

```
File : app\code\community\Keyur\RemoveCategoryTab\Model\Observer.php

class Keyur_RemoveCategoryTab_Model_Observer

{

public function removeUnwantedCategoryTabs(Varien_Event_Observer $observer)
{
    $tabs = $observer->getEvent()->getTabs();
  $tabs->removeTab('group_6');
    return $this;
}

```

}

Where `group_6` is tab id [You can check by inspecting element 

![enter image description here](https://i.stack.imgur.com/D9qbk.png)

And see the magic.

Let me know if you have any query
Not the best but simple way, use below property in your CSS : 

```
#category_info_tabs > li:nth-child(3) {
    display: none;
}

```