[Follow up] Clean-up tasks to do once direct desugaring is complete

[amomchilov]

Once we've fully finished implementing direct desugarings in Translator.cc (#444), we can clean up a whole bunch of stuff. Use this issue to track such tasks.

1. Remove all usages of Whitequark nodes
   • Remove legacy parse tree construction from the Prism translator sorbet/sorbet#9780
   • Part done in Clean up legacy parser AST roundtrip from Prism code path sorbet/sorbet#9999, merged in Hide the Translator class in the cc file sorbet/sorbet#10000
2. Delete PrismDesugar.cc, and rename Translator.cc to be the new PrismDesugar.cc
   • Opened as , but merged in Hide the Translator class in the cc file sorbet/sorbet#10000
3. Stop treating unsupported nodes as if they are self
   • Desugar unsupported nodes to error nodes instead of empty trees sorbet/sorbet#10030
4. Change desugarScopeBodyToRHSStore to not return std::optional
5. Stop skipping tests such as sorbet@9feb9fb exclusions in test/BUILD
6. Update this comment to not reference parser::Array and parser::Send
7. Narrow the return type of this function and others which had their types widened