[Themes] Remove legacy message in the `theme dev` command

[jamesmengo]

WHY are these changes introduced?

Fixes https://github.com/Shopify/develop-advanced-edits/issues/361

WHAT is this pull request doing?

Removes --legacy message when users run the new theme dev command

How to test your changes?

pnpm shopify theme dev should not render a message matching the screenshot in the issue

Measuring impact

How do we know this change was effective? Please choose one:

• [ ] n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• [x] Existing analytics will cater for this addition
• [ ] PR includes analytics changes to measure impact

Checklist

• [x] I've considered possible cross-platform impacts (Mac, Linux, Windows)
• [x] I've considered possible documentation changes