Run hugo mod vendor
and then search the code base for .AllTranslations.
You’ll find this file:
_vendor/github.com/gethugothemes/hugo-modules/seo-tools/basic-seo/layouts/partials/basic-seo.html
With this code:
<!-- multilingual SEO optimizations -->
{{ if .IsTranslated }}
{{ range .AllTranslations }}
<link rel="alternate" hreflang="{{ .Lang }}" href="{{ .Permalink }}" />
{{ end }}
<link rel="alternate" hreflang="x-default" href="{{ .Permalink }}" />
{{ end }}
Just override that partial (create layouts/partials/basic-seo.html), and remove the section above.
When you’re done, remove the _vendor directory.