In the drop-downs at the top of the page, you can go to Personal menu then the sub-menu for Pets. Click the "Pets" link below the name of the pet you want to remove the potions from. It will take you to a page with your pet's image and a list of options below. Click on the link that says "Edit Battle Weapons"...

Alternatively, you can copy and paste this link into your browser's address bar but you will have to find the pet's ID to use it. https://subeta.net/pet.php?act=battle&petid=ThePetIDGoesHere