Follow-up from "feat: Add markdown to ML Model descriptions" - remove full screen button

The following discussion from !158925 (merged) should be addressed:

• @jackib started a discussion: (+3 comments)

  @fdegier Can we have @grrbam do a UX review on this?

1. The full-screen button should be removed since the markdown editor is contained in a modal.

2. Increase the size of the modal to LG