How to display plain text only from redactor without any line break or format
If you're trying to remove HTML tags from the output completely, you can use Twig's striptags filter: https://twig.symfony.com/doc/2.x/filters/striptags.html.

If you want to retain the tags but show the output without any styling, you're going to have to handle that with your CSS by negating any of the existing styling you don't want.