Since [1], the fragment specifier is unconditionally required in all
editions. This means `NonTerminalKind` no longer needs to be optional,
as we can reject this code during the expansion of `macro_rules!` rather
than handling it throughout the code. Do this cleanup here.
[1]: https://github.com/rust-lang/rust/pull/128425
We are moving toward forbidding `missing_fragment_specifier` either in
edition 2024 or unconditionally. Make a first step toward this by
ensuring crates that rely on the old behavior are reported when used as
dependencies.
Tracking issue: <https://github.com/rust-lang/rust/issues/128143>