chore: cleanup Mistral Nemo references

What does this merge request do and why?

• The model is not being used. Removing support as there are better alternatives.
• From https://gitlab.com/gitlab-org/gitlab/-/issues/566739#note_2863893531

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.