Board Thread:Support Requests - Getting Technical/@comment-35616940-20190107200633/@comment-9605025-20190110003724

You delete a template from the wiki just as you would any other page. Go to the template's page, click the dropdown arrow next to the edit button, then click delete.

As for spanning the whole page, portable infoboxes are not meant to do that. If you want to create a template that spans the whole page, likely you should be creating "classic" template. If you are really sure that a portable infobox is the way to go, then you will need to modify the width via your wiki's custom CSS.