Remove Unused petit Gem Dependency

[Hamms]

Originally added in #17132 to enable us to build a url-shortening service. As far as I can tell, we never got around to building that service and currently don't have any functionality we provide which uses this gem. Removing it in preparation for entirely removing our dependency on unicorn, which this gem is entangled with.

Links

• Set up petit gem. #17132
• Set up shortener gem. #17102
• https://github.com/code-dot-org/petit

Testing story

Searched slack, github, and the repository itself for any other references to this gem or the functionality we intended to build on top of it. Found nothing

Follow-up work

We could consider eliminating our fork at https://github.com/code-dot-org/petit

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