Option 1

If you want to remove intro text in your entire site use this option...

Open templates > TEMPLATE > html > com_content > category > blog_item.php and delete/remove below line [Line 58 ]

<div class="article-introtext">
    <?php echo $this->item->introtext; ?>
</div>

Option 2

If you want to remove intro text in some selected Category (or Menu) use this option...

Go to the menu where you wanted to remove the intro text, and enter your custom class at Page Display > Page Class

Paster below CSS in your custom.css file...

.no-intro .article-introtext {
    display: none !important;
}