[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
2. Delete PrismDesugar.cc, and rename Translator.cc to be the new PrismDesugar.cc
3. Stop treating unsupported nodes as if they are self
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