Remove site name from template using twig function
You can remove the block in the block layout.

If you don't want to do it in the ui, you have to look for the block name. This is probably sitebranding. 

For example to remove the block with the name sitebranding from the region header in the page template:

{{ page.header|without('sitebranding') }}


If you are looking for the site name in the kint print out, it is not always possible to find the actual content early in the templates. Often it is processed at a later stage.