Tree traversal (`transformOther`) in `CheckUnused.scala` could need a clear refactor

As suggested by @smarter [#16684](https://github.com/lampepfl/dotty/pull/16684#issuecomment-1387181428), the tree traversal of [CheckUnused.scala](https://github.com/lampepfl/dotty/blob/main/compiler/src/dotty/tools/dotc/transform/CheckUnused.scala) could need a refactor, as it may appear confusing to some that different traversers are used. Maybe a better design can be proposed.