[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

[github-actions[bot]]

Thanks for your contribution!

Depending on what you are working on, you may want to request a review from a Shopify team:

• Themes: @shopify/advanced-edits
• UI extensions: @shopify/ui-extensions-cli
  • Checkout UI extensions: @shopify/checkout-ui-extensions-api-stewardship
• Hydrogen: @shopify/hydrogen
• Other: @shopify/app-inner-loop

[github-actions[bot]]

Coverage report

St.:grey_question:	Category	Percentage	Covered / Total
🟡	Statements	72.72% (+0.01% 🔼)	8562/11774
🟡	Branches	69.71% (+0.02% 🔼)	4204/6031
🟡	Functions	71.69%	2211/3084
🟡	Lines	73.05% (+0.01% 🔼)	8104/11094

Test suite run success

1947 tests passing in 876 suites.

Report generated by 🧪jest coverage report action from 2339529909659fc8a178893a98a07b5f24f9c0f2