We weren’t able to reproduce this on WordPress 5.7.2, Yoast SEO 16.4 and Web Stories 1.7.2. The meta robots for a specific web story and the web-story content type was checked and only one meta robot is outputted.
However, you asked if it is possible to remove the robots meta tag from Yoast. You can do this by navigating to WordPress > SEO > Search Appearance > Content Types. For the option, Show Stories in search results?, choose ‘No’. We do not think this will provide the desired results.
Often, we see problems occur in combination with another plugin or theme. The fastest way to rule out any conflict, is to deactivate all non-Yoast plugins and switch to a standard theme like Twenty Twenty.
Please test this on your development or staging site, if you have one. If not, we recommend using the Health Check & Troubleshooting plugin. This plugin has a troubleshooting mode, which does not affect normal visitors to your site.
If you’re unfamiliar with checking for conflicts, we’d like to point you to a step-by-step guide that will walk you through the process: How to check for plugin conflicts
If you do not feel comfortable doing this yourself or if this does not solve your issue, our Yoast SEO Premium plugin comes with one year of (technical) support.`