Simplify styleClass tests once d2 fmt bug re: no output if no change needed is fixed

[smheidrich]

See:

• TODO.md in this repo
• fmt from stdout should always output the formatted diagram terrastruct/d2#1577

[smheidrich]

@SnoozeThis terrastruct/d2#1577

[SnoozeThis]

(https://snoozeth.is/Wc0UHblZ4yU) I will wait until terrastruct/d2#1577 is closed and then add a comment.