[Themes] Remove Ruby from `theme pull` command

[jamesmengo]

WHY are these changes introduced?

Fixes https://github.com/Shopify/develop-advanced-edits/issues/351

WHAT is this pull request doing?

• Removes legacy flag from theme push
• Removes Ruby Invocation from theme push

How to test your changes?

• Try running with --legacy flag -> this shouldn't return an error message

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