While we're here, remove a number of `use` statements from generated
code. These statements are dangerous beacause they risk bringing items
into scope whose names shadow the identifiers which we have parsed from
the derive input and will emit into the scope. This will cause those
references to erroneously refer to the `use`'d items instead of their
intended referents.

gherrit-pr-id: G9c0c3d07b59c78d561a16ab42bfe6eacaadd7156