How to set a field value from parent template in Twig?
There should be no reason to change any code for this, as you are using the default image formatter. You should be able to remove the link by going to /admin/structure/types/manage/article/display/teaser and change your field's formatter settings using the cogwheel-button and then select "None" for "Link image to".

Should the teaser view mode configuration page not be available, got to /admin/structure/types/manage/article/display and activate its checkbox on the far bottom of the page, then save and try again.