Pimcore\Model\Element\Tag::removeTagFromElement PHP Method

    public static function removeTagFromElement($cType, $cId, Tag $tag)
    {
        $tag->getDao()->removeTagFromElement($cType, $cId);
    }

 public function removeTagFromElementAction()
 {
     $assginmentCId = intval($this->getParam("assignmentElementId"));
     $assginmentCType = strip_tags($this->getParam("assignmentElementType"));
     $tagId = intval($this->getParam("tagId"));
     $tag = Tag::getById($tagId);
     if ($tag) {
         Tag::removeTagFromElement($assginmentCType, $assginmentCId, $tag);
         $this->_helper->json(['success' => true, 'id' => $tag->getId()]);
     } else {
         $this->_helper->json(['success' => false]);
     }
 }