Remove Event Param from OnBoardEvent [Circuit Playground]

[epeach]

Extension of #43973 to apply to Circuit Playground blocks.

We want to remove the 'event' parameter from onBoardEvent since we don't require it to be used and it adds a linter warning to student code (yellow triangle). Per curriculum request.

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked