fix: remove flat polyfill, use Array.prototype.flat instead

[dnalborczyk]

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests * (already covered)
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.